2-NRLF 


LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


OTHER  WORKS 

BY 

J.    H.    K1NEALY. 


CENTRIFUGAL  FANS,  a  theoretical  and  prac- 
tical treatise  on  fans  for  moving  air  in  large 
quantities  at  comparatively  low  pressures,  prin- 
cipally compiled  from  the  author's  private  note- 
books, with  22  original  tables,  and  39  diagrams, 
full  limp  leather,  round  corners,  gilt  edges,  $5.00. 

AN  ELEMENTARY  TEXT-BOOK  ON  STEAM  EN- 
GINES AND  BOILERS,  for  the  use  of  students  in 
schools  and  colleges,  5th  edition,  267  pages,  109 
illustrations,  8vo,  cloth,  $2.00. 

FORMULAS  AND  TABLES  FOR  HEATING  AND 
VENTILATING  WORK,  flexible  leather,  $1.00. 

CHARTS  FOR  Low  PRESSURE  STEAM  HEATING 
for  the  •  use  of  Engineers,  Architects,  Con- 
tractors, and  Steam  Fitters,  printed  on  large 
folding  cards.  $1.00. 


MECHANICAL  DRAFT 

A  PRACTICAL  HANDBOOK 

FOR 

ENGINEERS  AND  DRAFTSMEN 


BY 


J.  H.  KINEALY, 


Formerly  Professor  of  Mechanical  Engineering  at  \Vashington 
University;  Member  American  Society  of  Mechanical  Engi- 
neers; Member  Society  of  Arts,  England;  Past-President  Amer- 
ican Society  of  Heating  and  Ventilating  Engineers;  Past- 
President  Engineers  Club  of  St.  Louis;  Fellow  American  As- 
sociation for  the  Advancement  of  Science;  etc.,  etc. 


WITH  TWENTY-SEVEN  ORIGINAL  TABLES 

AND 

THIRTEEN  HALF-TONE  PLATES 


NEW  YORK 
SPON  &  CHAMBERLAIN,  123  LIBERTY  ST., 

LONDON 

E.  &  F.  N.  SPON,  LIMITED,  57  HAYMARKET,  S.  W. 
1906 


Copyright  at  Washington,  D.  C. 
1906,  by  J.  H.  KINEAL*. 


Press  of  MclLROY  &  EMMET,  22  Thames  Street,  N.  Y.,  U.  S.  A. 


OF  THE 

UNIVERSITY 


PREFACE. 


In  writing  this  little  book  the  author  has  as- 
sumed that  those  who  will  use  it  are  familiar 
with  boiler  and  engine  plants,  and  he  has  had 
in  mind  the  practicing  engineer  who  is  called 
upon  to  design  power  plants,  and  who  must  there- 
fore decide  when  it  is  best  to  use  some  form  of 
mechanical  draft.  The  arrangement  of  the  book 
is  what  the  experience  of  the  author  in  making 
calculations  for  mechanical  draft  installations 
has  shown  him  is  probably  the  best.  And  he  has 
tried  to  arrange  the  tables  in  such  a  way  and  in 
such  a  sequence  that  they  may  prove  as  useful 
to  others  as  they  have  to  him. 

With  the  exception  of  such  tables  and  matter 
as  has  been  taken  from  the  author's  book  on 
Centrifugal  Fans,  all  of  the  tables  that  are  to  be 
used  in  designing  a  mechanical  draft  plant  are 
new  and  have  been  calculated  especially  for  this 
work.  Tables  have  been  used  liberally  because 


179741 


IV  PREFACE. 

they  have  been  made  for  the  conditions  which 
generally  occur  in  actual  practice  and  because 
they  decrease  so  enormously  the  labor  of  design- 
ing a  plant.  In  all  cases,  however,  the  formulas 
used  in  calculating  the  tables  have  been  given,  so 
that  for  all  those  conditions  that  are  beyond  the 
range  of  the  tables,  engineers  may  make  their 
own  calculations. 

While  the  book  is  intended  primarily  for  the 
practicing  engineer,  full  explanations  of  the  va- 
rious steps  leading  up  to  the  finished  design  of  a 
mechanical  draft  apparatus  have  been  given  with 
the  hope  that  the  book  may  prove  of  value  as  a 
text-book  for  young  engineers  and  students.  The 
author  can  call  to  mind  no  book  which  even  at- 
tempts to  discuss  the  subject  of  mechanical  draft 
in  such  a  way  as  to  enable  a  student  to  get  any 
idea  as  to  the  steps  to  be  followed  in  designing  a 
mechanical  draft  apparatus  so  that  it  will  give 
certain  predetermined  results  under  given  fixed 
conditions,  and  he  is  led,  therefore,  to  hope  that 
the  book  will  find  favor  with  the  teachers  of  en- 
gineering as  well  as  with  their  students  and  the 
older  practicing  engineers. 

The  method  of  treatment  is  new,  the  equations 
are  new,  the  tables  are  new ;  and  the  book  is  a 
monograph  representing  years  of  study  and  work 


PREFACE.  V 

on  the  part  of  the  author.  And  last  but  not  least 
is  the  fact  that  the  book  has  been  written  and  has 
not  been  made  from  the  books  of  others  by  a  free 
use  of  the  scissors. 

The  author  has  striven  to  add  something  to 
the  sum  total  of  the  engineering  knowledge  of 
the  world,  and  if  he  has  been  able  to  do  this  he 
will  not  consider  that  his  efforts  have  been  in 
vain. 

The  author  desires  to  express  his  thanks  to 
The  Green  Fuel  .Economizer  Co.,  The  American 
Blower  Co.,  B.  F.  Sturtevant  Co.,  and  the  Niag- 
ara Radiator  Co.,  for  their  kindness  in  supplying 
the  photographs  used  for  most  of  the  illustra- 
tions. 

J.    H.   KlNEALY. 

February,  /pod. 
St.  Louis,  Mo. 


CONTENTS  OF  CHAPTERS. 


CHAPTER  I. 

GENERAL  DISCUSSION. 

PAGE 

Introduction     i 

Systems   of   mechanical    draft 4 

Chimneys  vs.  mechanical  draft 5 

Liability  to  derangement 6 

First    cost 7 

Depreciation  and  repairs 12 

Running    expenses 12 

Economy  in  operation  of  plant 14 

Future    increase    of   plant 16 

High   draft. 17 

Mechanical  draft  and  economizers 0 .  18 

CHAPTER  II. 

FORCED  DRAFT. 

Systems    20 

Closed   fire-room  system 21 

Closed  ash-pit  system 22 

Small   fan   required 23 

Usual    pressure 24 

Forced  draft  and  economizers 26 

vii 


Vlll  CONTENTS. 

PAGE 

Advantages     26 

Disadvantages    27 

CHAPTER  III. 
INDUCED    DRAFT. 

Introduction 30 

Temperature   of  gases 32 

Advantages  35 

Disadvantages    38 

CHAPTER   IV. 
FUEL  AND  AIR. 

Weight  of  coal  to  be  burned 41 

Evaporation   per  pound  of  coal 42 

Effect  of  rate  of  evaporation 43 

Weight  of  air  required 46 

Volume  of  air  and  gases 47 

Volume  of  gases  to  be  handled 48 

Leakage 50 

Factor  of  safety 51 

CHAPTER   V. 
DRAFT. 

Relation  to  rate  of  combustion : 52 

Resistance  of  grate 60 

Resistance  due  to  economizer 63 

Draft  required  under  Different  conditions 66 


CONTENTS.  IX 

CHAPTER  VI. 
ECONOMIZERS. 

PAGE 

Effect   of  adding 70 

Ordinary  proportion  and  cost , 74 

Increase  of  temperature  of  feed  water 75 

CHAPTER  VII. 

FANS.  . 

Type  and  proportions  of  fan  used 8r 

Relation  between  revolutions  of  fan  and  draft 86 

Capacity   of  fan 89 

CHAPTER  VIII. 
PROPORTIONING  THE  PARTS. 

Diameter  of  fan  wheel  required 92 

Speed  at  which  the  fan  must  be  run ,    ...     97 

Power  required  to  run  the  fan 98 

Size   of   engine    required 107 

Steam  used  by  fan  engine in 

Choosing  the  fan    in 

Choosing  the  fan  for  forced  draft 114 

Choosing  the  fan  for  induced  draft  with  economizer  115 
Choosing  the  fan  for  induced    draft   without    econ- 
omizer    116 

Location  of  the  fan 117 

Breeching   and   uptake 117 

Inlet   chamber..  .   122 


X  CONTENTS. 

PAGE 

Discharge    chimney 122 

By-pass    124 

Water  for  bearings .   126 

Appendix.     General    tables 129 


LIST  OF  TABLES. 


PAGE 

I.     Maximum  evaporation  for  different  coals.  44 

II.     Value  of  z  for  different  boilers 45 

III.  Value  of  volume  factor 48 

IV.  v  Whitham's    tests 57 

V.     Whitham's    tests 58 

VI.     Wagner's  tests 60 

VII.     Whitham's  tests,  resistance  of  grate 62 

VIII.     Induced     draft     necessary     without     an 

economizer    66 

IX.     Induced   draft   necessary   with   an    econ- 
omizer      67 

X.     Forced  draft,  pressure   necessary,   closed 

ash-pit  system    69 

XI.     Roney's  experiments  with  economizers..  77 

XII.     Economizer    factors 79 

XIII.  Size  of  fan  necessary  for  forced  draft.  .  94 

XIV.  Size  of  fan  necessary  for  induced  draft 

with  an  economizer 95 

XV.     Size  of  fan  necessary  for  induced  draft 

without  an  economizer 96 

XVI.     Revolutions  per  minute  of  fan  wheel  for 

forced    draft 99 

XVII.     Revolutions  per  min'ute  of  fan  wheel  for 

induced  draft  with  an  economizer 100 

xi 


Xll 


LIST    OF    TABLES. 


PAGE 


XVIII.     Revolutions  per  minute  of  fan  wheel  for 

induced  draft  without  an  economizer. .   101 
XIX.     Horse  power  required  for  fans  for  forced 

draft 104 

XX.     Horse  power   required   for   fans   for   in- 
duced draft  with  an  economizer 105 

XXI.     Horse  power   required   for   fans    for   in- 
duced draft  without  an  economizer....   106 
XXII.     Ratio    diameter    of    engine    cylinder    to 

diameter  of  wheel no 

XXIII.  Breeching    and    uptake    connections    for 

induced    draft 123 

XXIV.  Dimensions    of    full    housed,    horizontal 

discharge    fans    131 

XXV.     Thickness  of  iron 132 

XXVI.     Areas  of  circles 133 

XXVII.     Sizes   of   engines    suitable   for   use   with 

mechanical   draft   installations 134 


LIST  OF  PLATES. 


PLATE.  FACING   PAGE 

I.     Bottom,  Horizontal  Discharge  Fan 5 

(Niagara  Radiator  Co.) 

II.     Double    Discharge    Fan.... 15 

(Niagara   Radiator   Co.) 

III.  Induced  Draft  Apparatus,  Washington  Uni- 

versity,  St.  Louis,  Mo 25 

IV.  Induced    Draft    Apparatus,    Victoria    Hotel, 

New   York    35 

(American  Blower  Co.) 

V.     Economizer  as  Applied  to  Power  Plants....     45 
(Green   Fuel   Economizer  Co.) 

VI.  Induced     Draft     Apparatus,     Neptune     Con- 

sumers Ice  Co.,  Brooklyn,  N.  Y 55 

(American  Blower  Co.) 

VII.  Economizer,     Atlanta     Consolidated     Street 

Railway  Co.,   Atlanta,   Ga 65 

('Green  Fuel  Economizer  Co.) 


LIST    OF    PLATES. 

VIII.     Induced     Draft     Apparatus,     State     Central 

Heating   Plant,  Jefferson   City,   Mo 75 

(Niagara  Radiator   Co.) 

IX.     Economizer,     Clark    Thread    Co.,     Newark, 

N.    J 85 

('Green   Fuel   Economizer  Co.) 

X.     Induced    Draft   Apparatus,    Bay   City   Trac- 
tion &  Electric  Co.,  Bay  City,  Mich 95 

(B.   F.    Sturtevant  Co.) 

XL     Induced    Draft    Apparatus,    Hudson    River 

Electric  Power  Co.,  Utica,  N.  Y 105 

(American  Blower  Co.) 

XII.     Economizer   and   Induced   Draft   Apparatus, 

B.  F.  Sturtevant  Co.,  Hyde  Park,  Mass...   115 
(B.   F.    Sturtevant  Co.) 


CHAPTER  I. 

GENERAL   DISCUSSION. 

Introduction.  When  laying  out  a  power  plant 
the  engineer  must  determine  the  draft  necessary 
to  burn  the  required  amount  of  fuel  per  hour, 
and  also  how  this  draft  shall  be  created,  whether 
it  shall  be  natural  or  artificial.  A  draft  is  said 
to  be  natural  or  chimney  draft  when  it  is  due  to 
the  difference  between  the  density  of  the  cold  air 
on  the  outside  of  a  chimney  and  that  of  the  hot 
products  of  combustion  passing  upward  on  the 
inside,  and  a  draft  is  said  to  be  artificial  when  it 
is  produced  artificially  by  means  of  a  jet  of  steam 
or  some  form  of  blower  or  fan. 

If  the  draft  is  produced  by  some  form  of  fan 
or  blower  it  is  said  to  be  mechanical  draft. 

The  draft  produced  by  a  chimney  depends 
upon  the  height  of  the  chimney,  the  temperature 
of  the  hot  gases  inside,  and  the  temperature  of 
the  cold  air  outside.  It  may  be  affected  by  the 
direction  and  velocity  of  the  wind,  and  by  the 


2  MECHANICAL    DRAFT. 

humidity  of  the  outside  air,  but  as  the  influence 
of  either  of  these  is  usually  small  they  are  always 

^jieglected  when  designing  a  power  plant.  When 
the  draft  is  expressed  in  inches  of  water  it  is 
found  that  the  draft  created  by  a  chimney  varies, 
according  to  the  temperature  of  the  hot  gases  and 
that  of  the  air  outside,  from  0.005  to  0.007  times 

f  .  the  height  of  the  chimney  in  feet.  The  tempera- 
ture of  the  gases  passing  out  through  a  chimney 
is  greater  at  the  base  than  at  the  top,  and  it  is 
the  average  temperature  which  determines  the 
draft  which  will  be  created  for  a  given  height  of 
chimney  and  a  given  outside  temperature.  As  it 
is  practically  impossible  to  measure  the  tempera- 
ture of  the  gases  at  different  points  from  the 
base  to  the  top,  it  is  impossible  to  determine  the 
average  temperature  of  the  gases  in  a  chimney. 
The  temperature  of  the  gases  in  a  chimney  is 
always  measured  near  the  base,  sometimes  in  the 
chimney  itself  and  other  times  in  the  uptake  or 
breaching  between  the  boiler  and  the  chimney. 
The  higher  the-  chimney  the  greater  is  the  fall  of 
temperature  as  the  gases  pass  upward,  and  hence 
the  less  is  the  average  temperature  of  the  gases 
in  the  chimney,  and,  therefore,  the  greater  is  the 
difference  between  the  draft  actually  created  and 
the  draft  calculated  upon  the  supposition  that  the 


GENERAL   DISCUSSION.  3 

temperature  at  the  base  is  the  average  tempera- 
ture. Since  the  cooling  of  the  gases  passing  up- 
ward is  greater  in  a  steel  or  an  iron  chimney 
than  in  a  thick,  heavy  brick  ^chimney,  the  draft 
created  by  a  steel  or  an  iron  chimney  is  in  gen- 
eral less  for  a  given  temperature  of  the  gases  at 
the  base  and  a  given  temperature  of  the  outside 
air,  than  for  a  brick  chimney  of  the  same  height. 

The  temperature  of  the  gases  measured  at  the 
base  of  a  chimney  varies  from  350  to  600  degrees 
Fahrenheit,  for  the  usual  boiler  plant  without 
economizers  or  an  air  heater  in  the  fireeching. 
Of  the  heat  carried  up  the  chimney  by  the  escap- 
ing gases,  only  that  which  is  not  necessary  to 
produce  the  required  draft  can  be  said  to  be  lost ; 
the  rest  is  used  in  creating  the  draft,  and  thus 
serves  an  exceedingly  useful  purpose. 

Steam  jets  for  the  creation  of  an  artificial  draft 
are  used  on  all  locomotives,  many  launches  and 
small  boats,  and  to  a  limited  extent  only  on  power 
plants.  As  used  on  power  plants,  the  steam  jet 
serves  usually  for  introducing  a  supply  of  air 
above  the  grate  bars ;  and  is  generally  used  more 
for  reducing  the  amount  of  smoke  by  serving  to 
mix  up  and  bring  about  a  more  complete  min- 
gling of  the  hot  gases  and  air  in  the  furnace  than 
for  increasing  the  draft. 


4  MECHANICAL    DRAFT. 

Mechanical  draft  is  used  on  almost  all  large 
steam-ships  and  to  a  considerable  extent  in  pow- 
er plants  on  land.  It  is  used  in  almost  all  cases, 
except  on  locomotives  and  launches,  where  it  is 
necessary  to  have  a  high  draft,  and  its  use  in 
connection  with  power  plants  both  large  and 
small  is  increasing  daily.  The  form  of  blower 
used  is  almost  invariably  the  ordinary  centrifugal 
fan,  and  the  draft  which  can  be  created  depends 
almost  entirely  upon  the  velocity  in  feet  per  min- 
ute of  the  periphery  of  the  blades.  So  that  by 
running  the  fan  fast  or  slow  the  draft  may  be 
increased  or  decreased  at  will,  and  thus  be  made 
to  suit  the  particular  requirements  of  the  plant 
at  any  time. 

Systems  of  Mechanical  Draft.  Mechanical 
draft  is  subdivided  into  two  systems  according 
to  whether  the  air  is  made  to  enter  the  furnace 
by  reducing  the  pressure  at  some  point  beyond 
the  boiler  so  as  to  make  it  less  than  atmospheric 
pressure  and  thus  suck  the  air  into  the  furnace, 
or  whether  the  air  is  forced  into  the  furnace  un- 
der a  pressure  slightly  greater  than  that  of  the 
atmosphere.  When  the  air  is  sucked  into  the 
furnace  and  enters  under  atmospheric  pressure, 
the  system  is  called  an  "  induced  draft  system  " ; 


pIG    i — BOTTOM,  HORIZONTAL  DISCHARGE  FAN. 
(Niagara  Radiator  Co.) 


GENERAL    DISCUSSION.  5 

and  when  the  air  is  forced  into  the  furnace,  the 
system  is  called  a  "  forced  draft  system/' 

The  same  kind  of  fan  is  used  for  both  systems, 
but  the  details  of  the  two  systems  differ  in  many 
respects,  and  it  often  happens  that  one  is  much 
more  suitable  for  certain  conditions  than  the 
other.  Except  in  connection  with  certain  types 
of  patented  mechanical  stokers,  the  forced  draft 
system  is  not  used  as  much  with  power  plants  as 
the  induced  draft  system,  while  on  steam-ships 
the  forced  system  is  used  almost  entirely  to  the 
complete  exclusion  of  the  induced  system. 

Chimney  vs.  Mechanical  Draft.  It  is  prob- 
able that  after  considering  this  question  the  en- 
gineer will  find  that  the  real  decision  must  be 
between  chimney  or  natural  draft  and  mechanical 
draft ;  and  as  the  mechanical  systems  of  draft  are 
newer  and  less  tried  than  the  chimney  draft,  he 
will  look  for  the  advantages  which  it  is  claimed 
mechanical  draft  has  over  chimney  draft  and 
weigh  them  with  the  disadvantages. 

In  comparing  chimney  with  mechanical  draft 
the  practical  engineer  would  be  likely  to  consider 
each  under  the  following  heads : 

i. —  Liability  to  derangement. 
2. —  First  cost. 


O  MECHANICAL    DRAFT. 

3. —  Depreciation  and  repairs. 

4. —  Running  expenses. 

5. —  Economy  in  operation  of  plant. 

6. —  Provision  for  future  increase  of  plant. 

Liability  to  Derangement.  As  far  as  liability 
to  derangement  is  concerned,  every  engineer 
knows  that  there  is  nothing  about  a  chimney  to 
get  out  of  order,  no  machinery  of  any  kind  and 
no  moving  parts,  and  the  only  way  a  chimney 
can  be  put  out  of  service  is  for  it  to  fall.  Thin, 
guyed,  sheet  steel  chimneys  rust  out  quite  rap- 
idly and  then  are  easily  blown  over ;  self-sup- 
porting steel  chimneys,  either  lined  or  unlined, 
usually  have  a  much  longer  life,  the  length  of 
which  depends  naturally  upon  the  thickness  of 
the  metal  of  which  they  are  made  and  the  care 
given  to  them ;  brick  or  stone  chimneys  when 
well  built  last  practically  forever,  and  when  prop- 
erly designed  and  erected  do  not  fall  unless  struck 
by  lightning  or  a  cyclone.  A  mechanical  draft 
apparatus,  however,  always  comprises  in  addi- 
tion to  the  fan  or  blower  a  motor  of  some  kind 
for  driving  the  fan  or  blower ;  so  that  there  are 
many  moving  parts,  any  one  of  which  is  liable  to 
give  trouble.  In  fact,  a  mechanical  draft  appa- 
ratus is  a  machine,  liable  to  all  the  accidents  and 
ills  of  a  simple  machine ;  and  because  of  this  fact 


GENERAL   DISCUSSION.  7 

it  is  necessary,  when  the  draft  depends  entirely 
upon  the  mechanical  draft  apparatus  and  there 
is  no  chimney  to  fall  back  upon  in  case  of  an 
accident  to  the  fan  or  its  motor,  to  install  dupli- 
cate fans  and  motors.  When,  however,  there  are 
duplicate  fans  and  motors  of  the  proper  size, 
there  is  no  more  danger  of  a  mechanical  draft 
apparatus  being  put  entirely  out  of  service,  or 
becoming  so  deranged  as  to  cause  a  shutdown  of 
the  entire  power  plant  than  there  is  in  the  case 
of  a  chimney  of  brick  or  stone. 

First  Cost.  It  is  extremely  difficult  to  make  a 
general  comparison  of  the  first  cost  of  a  chimney 
with  that  of  a  mechanical  draft  plant,  because  of 
the  fact  that  most  chimneys  for  power  plants  are 
usually  put  up  with  a  view  of  obtaining  a  draft 
from  0.5  to  0.75  of  an  inch  of  water,  while  me- 
chanical draft  systems  are  seldom  installed  except 
to  give  a  draft  of  not  less  than  at  least  one  inch. 
It  is  probable  that  most  chimneys  are  between 
TOO  ami  150  feet  high,  while  a  chimney  to  give 
a  draft  of  one  inch  would  have  to  be  between 
175  and  200  feet  high,  and  the  cost  of  a  chimney 
increases  very  much  as  the  height  is  made  great- 
er than  about  125  feet.  Moreover,  for  a  large 
power  plant  several  small  steel  chimneys  are 


O  MECHANICAL   DRAFT. 

often  put  up  instead  of  one  large  brick  chimney, 
and  these  chimneys  may  be  of  cheap  steel  con- 
struction, so  that  the  cost  may  be  small.  Again, 
there  is  the  curious  difference  between  a  chimney 
and  a  mechanical  draft  apparatus,  that  while  a 
tall  chimney  to  give  a  high  draft  costs  more  than 
a  low  chimney  to  burn  the  same  quantity  of  coal 
under  a,  low  draft,  a  fan  to  supply  air  for  a  given 
quantity  of  coal  under  a  high  draft  costs  less 
than  a  fan  for  the  same  quantity  of  coal  under  a 
low  draft.  .  A  low  draft,  however,  means  a  low 
rate  of  combustion  per  square  foot  of  grate  sur- 
face, and  hence  a  large  area  of  grate  in  order  to 
burn  a  given  quantity  of  coal  per  hour,  and  it 
means  also  an  almost  total  inability  to  burn  coals 
of  very  low  grade ;  while  a  high  draft  means  a 
rapid  combustion  per  square  foot  of  grate  sur- 
face, and  hence  a  small  grate  to  burn  a  given 
quantity  of  coal  per  hour,  and  also  the  ability  to 
burn  cheap  coals  of  low  grade. 

A  chimney  to  give  a  draft  of  0.75  of  an  inch 
must  be  about  125  feet  high,  and  one  to  give  a 
draft  of  1.5  inches  would  probably  have  to  be  at 
least  250  feet  high.  The  cost  of  the  higher 
chimney  would  be  so  very  much  greater  than 
that  of  the  lower  that  few  engineers  would  rec- 
ommend it  solely  because  of  the  greater  draft 


GENERAL   DISCUSSION.  9 

which  could  be  obtained  with  it.  In  the  case  of 
a  mechanical  draft  apparatus,  however,  the  appa- 
ratus to  supply  the  air  for  the  combustion  of  a 
given  quantity  of  coal  per  hour  under  a  maximum 
draft  of  0.75  of  an  inch  would  be  larger  and  cost 
more  than  the  apparatus  to  supply  the  air  for  the 
combustion  of  the  same  quantity  of  coal  under  a 
maximum  draft  of  1.5  inches.  The  diameter  of 
the  fan  wheel  for  the  higher  draft  would  be  only 
about  0.83  of  the  diameter  of  the  fan  wheel  for 
the  lower  draft,  and  the  dimensions  of  the  engine, 
assuming  it  to  be  direct  connected  to  the  fan, 
might  also  be  smaller  for  the  higher  draft.  The 
work  done  in  running  the  fan  for  the  higher 
draft  would  be  twice  as  great  as  that  for  the 
lower,  and  hence  the  running  expenses  would  be 
almost  twice  as  great,  but  even  then  the  running 
expenses  would  be  small.  Thus  to  supply  air  for 
the  combustion  of  5,000  pounds  of  coal  per  hour 
with  an  economizer  under  a  maximum  draft  of 
0.75  of  an  inch  of  water,  an  induced  mechanical 
draft  apparatus  would  require  a  fan  with  a  7- 
foot  wheel ;  while  to  supply  air  for  the  combus- 
tion of  the  same  quantity  of  coal  under  a  maxi- 
mum draft  of  1.5  inches  a  fan  with  a  wheel  6 
feet  in  diameter  would  be  more  than  ample  and 
a  5^ -foot  wheel  would  be  almost  large  enough. 


IO  MECHANICAL    DRAFT. 

The  7-foot  fan  would  have  to  be  run  at  a  speed 
of  195  to  200  revolutions  per  minute  and  would 
require  a  direct  connected  engine  having  a  cyl- 
inder 6  inches  in  diameter  with  an  8-inch  stroke ; 
while  the  6-foot  fan  would  have  to  be  run  at  a 
speed  of  about  325  revolutions  per  minute,  and 
an  engine  having  a  cylinder  6  inches  in  diam- 
eter and  an  8-inch  stroke  would  be  more  than 
ample  for  it,  because  of  the  greater  number  of 
revolutions  made  per  minute.  The  dimensions 
of  the  engine  are  based  upon  the  supposition  that 
the  boiler  pressure  would  be  at  least  100  pounds. 
This  example  illustrates  the  curious  anomaly  in 
regard  to  the  difference  between  a  chimney  and 
a  mechanical  draft  plant.  In  the  case  of  the 
chimney  the  consideration  of  first  cost  makes  the 
engineer  keep  the  chimney  as  low  as  possible  and 
get  along  with  as  low  a  draft  as  possible ;  while 
in  the  case  of  a  mechanical  draft  apparatus  the 
consideration  of  first  cost  makes  the  engineer 
keep  the  draft  as  high  as  possible.  The  running 
expense  is  what  makes  the  engineer  keep  the  draft 
given  by  a  mechanical  draft  apparatus  low,  but 
this  running  expense  is  usually  more  than  offset 
by  such  advantages  as  the  ability  to  burn  cheaper 
fuel  and  to  maintain  a  hotter  fire  in  the  furnace, 
thus  securing  that  more  perfect  combustion  for 


GENERAL   DISCUSSION.  II 

poor  fuels  which  always  attends  a  high  draft. 
It  is  seldom  that  a  mechanical  draft  system  is 
installed  to  give  a  draft  no  greater  than  would 
be  likely  to  be  given  by  a  chimney,  and  hence  the 
higher  draft  capable  of  being  obtained  with  the 
mechanical  draft  apparatus  must  be  carefully 
borne  in  mind  when  considering  the  first  cost. 
It  is  possible,  of  course,  to  put  up  one  or  more 
cheap  chimneys  for  a  power  plant  and  make  the 
cost  of  them  less  than  the  cost  of  a  properly 
designed  mechanical  draft  system,  but  it  is  prob- 
able that  in  most  cases  a  lined,  self-supporting 
steel  chimney  or  a  brick  chimney  will  cost  con- 
siderably more  than  a  mechanical  draft  apparatus 
capable  of  furnishing  air  for  the  combustion  of 
the  same  quantity  of  coal  per  hour,  and  further, 
capable  of  giving  a  higher  draft  than  the  chim- 
ney. When  because  of  local  conditions  it  is  nec- 
essary to  discharge  the  gases  of  combustion  at  a 
considerable  height,  100  or  150  feet  above  the 
ground,  the  mechanical  draft  apparatus  plus  the 
chimney  for  the  discharge  of  the  gases  may  cost 
even  more  than  a  chimney  alone  that  would  be 
capable  of  furnishing  at  a  low  draft  the  air  re- 
quired for  the  combustion  of  the  coal ;  but  if  the 
draft  required  be  at  all  high,  it  is  probable  that 
even  under  these  circumstances  the  cost  of  a  suit- 


12  MECHANICAL    DRAFT. 

able   mechanical   draft   apparatus   would  be   less 
than  that  of  the  chimney. 

Depreciation  and  Repairs.  The  yearly  sum 
to  be  set  aside  for  depreciation  and  repairs  for 
a  brick  chimney  may  be  considered  as  nothing ; 
while  in  the  case  of  a  fairly  good  sheet  steel 
chimney,  so  thin  as  to  require  guys,  it  will  prob- 
ably be  15  or  20  per  cent  of  the  first  cost;  and 
for  a  self-supporting  steel  chimney  it  will  depend 
entirely  upon  the  thickness  of  the  original  metal 
and  whether  or  not  the  chimney  be  lined.  The 
depreciation  and  repairs  of  a  mechanical  draft 
system  will  be  somewhere  between  10  and  15  per 
cent  of  the  first  cost. 

Running  Expenses.  The  running  expenses  of 
a  chimney  are  zero,  and  the  running  expenses  of 
a  mechanical  draft  plant  include  the  cost  of  at- 
tendance, oil  for  lubricating  the  bearings  of  the 
motors  and  the  fans,  steam  for  supplying  power 
to  the  motors,  and,  possibly  in  the  case  of  an  in- 
duced draft  system,  water  for  cooling  the  bear- 
ings of  the  fans.  The  attendance  required  is  usu- 
ally so  little  that  its  cost  may  be  neglected  as 
insignificant.  In  the  case  of  a  power  plant  using 
non-condensing  engines  and  exhausting  into  the 


GENERAL    DISCUSSION.  13 

atmosphere  the  water  used  for  cooling  the  bear- 
ings of  the  fans  of  an  induced  draft  apparatus 
may  be  run  to  the  feed  water  heater  and  used  as 
feed  water ;  but  when  non-condensing  engines 
are  used  and  the  exhaust  is  used  for  heating  pur- 
poses and  then  returned  to  the  feed  water  heater 
as  water,  or  where  condensing  engines  are  used, 
the  cost  of  the  water  used  to  cool  the  bearings  of 
the  fans  of  an  induced  draft  system  must  be 
charged  to  the  running  expense  of  the  apparatus. 
The  steam  used  by  the  engine  required  to  run  a 
fan  will  of  course  depend  upon  the  work  the  en- 
gine must  do  as  well  as  the  type  of  the  engine. 
The  work  to  be  done  depends  upon  the  volume 
of  air  to  be  handled  per  minute  and  the  maxi- 
mum draft  under  which  it  must  be  moved.  The 
engine  is  usually  connected  directly  to  the  shaft 
of  the  fan  and  is  not  very  efficient.  For  the  fan 
with  the  7-foot  wheel,  considered  before,  it  would 
be  necessary  to  use  an  engine  which,  if  direct 
connected  to  the  fan,  would  develop  something 
less  than  10  indicated  horse-power  when  taking 
steam  at  about  100  pounds  gauge  pressure  and 
making  between  195  and  200  revolutions  per 
minute,  and  would  probably  use  between  400  and 
500  pounds  of  steam  per  hour.  The  weight  of 
steam  which  would  be  evaporated  under  actual 


14  MECHANICAL   DRAFT. 

conditions  by.  the  combustion  of  the  coal  for 
which  the  fan  would  supply  air  would  probably 
be  between  30,000  and  40,000  pounds.  That  is 
the  weight  of  steam  used  by  the  mechanical  draft 
apparatus  would  be  between  i  and  1.67  per  cent 
of  the  steam  generated  by  the  plant.  If  the 
smaller  fan,  that  is  the  one  with  the  6- foot  wheel, 
were  used  and  the  air  handled  under  a  draft  at 
the  fan  of  1.5  inches,  the  weight  of  steam  used 
per  hour  would  probably  be  between  2  and  3.5 
per  cent  of  the  steam  generated  by  the  plant. 
Thus  it  is  seen  that  the  consideration  of  running 
expense  tends  to  mafce  the  engineer  adopt  a  low 
draft  rather  than  a  high  one  when  designing  a 
mechanical  draft  plant.  It  is  probable  that  the 
steam  used  by  the  engine  of  a  properly  designed 
mechanical  draft  apparatus  will  seldom  exceed 
3  or  3-5  Per  cent  °f  tne  total  steam  generated  by 
the  plant. 

Economy  in  Operation  of  Plant.     It  is  in  the 

economy  in  the  operation  of  the  plant  that  a 
mechanical  draft  apparatus  really  makes  its  great 
showing.  In  the  case  of  a  chimney  the  draft 
depends  upon  the  height  of  the  chimney,  the 
temperature  of  the  hot  gases  inside,  and  the  tem- 
perature and  condition  of  the  air  outside  of  the 


OF  THE 

JM1VERSITY 

or 


FIG.  2. — DOUBLE  DISCHARGE  FAN. 
(Niagara  Radiator  Co.) 


GENERAL   DISCUSSIOX.  15 

chimney,  while  in  the  case  of  a  mechanical  draft 
apparatus  it  depends  only  upon  the  power  of  the 
motor  to  run  the  fan;  if  a  high  draft  is  desired 
it  is  run  faster  and  if  a  low  draft  is  desired  it  is 
slowed  down.  Unless  the  chimney  be  built  very 
high  the  draft  produced  by  it  cannot  be  very 
great  and  the  range  of  draft  therefore  cannot  be 
great ;  further,  the  draft  can  only  obtain  its  maxi- 
mum when  the  fire  in  the  furnace  is  hot,  so  as 
to  make  the  gases  in  the  chimney  have  a  high 
temperature.  In  the  case  of  a  mechanical  draft 
apparatus  the  draft  can  be  increased  by  speeding 
'up  the  fan,  without  regard  to  the  condition  of 
the  fire  in  the  furnace.  In  the  case  of  a  chimney 
the  draft  can  be  increased  only  as  the  increase 
in  the  combustion  takes  place,  and  this  combus- 
tion is  due  to  the  draft.  The  two  are  dependent 
on  one  another  in  such  a  way  that  it  is  quite  im- 
possible to  increase  either  suddenly.  While  in 
the  case  of  the  mechanical  draft  it  is  possible  to 
suddenly  increase  the  draft  irrespective  of  the 
condition  of  either  the  fire  or  the  draft  previous- 
ly, and  this  increase  in  draft  is  at  once  followed 
by  an  increase  in  the  combustion  in  the  furnace. 
Again,  the  temperature  of  the  air  outside  may 
have  a  material  affect  on  the  draft  of  a  chimney, 
while  it  cannot  affect  at  all  the  draft  of  a  me- 


l6  MECHANICAL   DRAFT. 

chanical  draft  apparatus.  And  further,  the  high- 
er draft  that  can  be  created  by  means  of  the 
mechanical  draft  apparatus  enables  a  poorer 
grade  of  coal  to  be  used  than  could  be  burned 
with  the  chimney  draft,  and  this  usually  results 
in  ?  marked  reduction  in  the  running  expense. 
In  the  case  of  a  high  draft  the  rate  of  combus- 
tion is  higher  than  in  the  case  of  a  low,  and  this 
results  in  a  hotter  fire  and  therefore  for  some  fuels 
a  more  perfect  combustion  of  the  fuel  in  the  fur- 
nace. It  has  also  been  found  that  less  air  is  re- 
quired for  the  complete  combustion  of  a  pound 
of  coal  when  the  draft  is  high  than  when  it  is 
low,  and  this  means,  therefore,  an  economy  for 
the  higher  draft,  because  of  the  smaller  amount 
of  heat  carried  away  by  the  hot  gases. 

Future  Increase  of  Plant.  In  regard  to  the 
provision  for  future  increase  of  the  power  plant 
there  is  no  question  but  that  the  mechanical  draft 
plant  has  all  the  advantage.  When  a  chimney 
is  built  it  must  be  built  very  much  larger  than 
needed  in  order  t©  allow  for  future  growth,  and 
this  means  always  a  greater  first  cost  than  nec- 
essary ;  and  when  the  plant  has  grown  so  that 
the  chimney  has  reached  its  limit  of  capacity  it 
then  becomes  necessary  to  build  a  new  chimney. 


GENERAL   DISCUSSION.  I/ 

It  is  because  of  this  continual  growth,  the  rate 
of  which  cannot  be  foreseen,  that  many  plants 
are  equipped  with  several  cheap  steel  chimneys, 
each  added  as  the  increase  of  the  plants  necessi- 
tates it,  rather  than  one  large  brick  chimney.  In 
the  case  of  a  mechanical  draft  apparatus  the 
capacity  of  the  plant  can  be  very  much  increased 
simply  by  speeding  up  the  fan,  and  when  this 
has  been  done  as  much  as  is  advisable  or  eco- 
nomical, it  is  cheaper  to  make  an  addition  to  the 
mechanical  draft  apparatus  than  it  would  be  to 
put  up  a  chimney  capable  of  giving  the  same 
draft  and  handling  the  products  of  combustion 
from  the  same  quantity  of  coal. 

High  Draft.  The  advantages  of  mechanical 
draft  are  all  due  to  the  high  draft  which  always 
accompanies  it,  and  to  the  ability  to  make  the 
draft  suit  the  requirements  of  the  plant  without 
regard  to  the  temperature  of  the  hot  gases  or 
that  of  the  outside  air.  In  most  cases  it  will  be 
found  that  it  is  possible  to  attain  the  high  draft 
and  the  attending  advantages  for  a  less  expendi- 
ture of  money  per  year,  including  interest  on 
first  cost  and  depreciation  and  repairs  of  the  draft 
producing  apparatus,  by  means  of  a  properly 
designed  mechanical  draft  apparatus  than  by 


l8  MECHANICAL   DRAFT. 

means  of  a  chimney.  And,  further,  by  making 
the  fans  and  motors  in  duplicate  the  danger  of  a 
shutdown  of  the  power  plant  because  of  some 
derangement  of  a  fan  or  its  motor  may  be  en- 
tirely eliminated. 

Mechanical  Draft  and  Economizers.  It  will 
be  noticed  that  in  what  has  been  said  no  men- 
tion has  been  made  of  an  economizer  to  be  used 
in  conjunction  with  the  boiler  plant.  This  course 
has  been  taken  because  an  economizer  can  be 
used  with  a  chimney  just  as  well  as  with  a  me- 
chanical draft  apparatus,  provided  always .  the 
chimney  is  of  such  a  height  that  it  will  give  the 
required  draft  when  the  economizer  is  used. 
There  are  any  number  of  power  plants  that  have 
economizers  where  the  draft  is  created  by  a  tall 
chimney,  and  the  economizers  do  their  work  well 
and  are  a  source  of  economy.  An  economizer 
ley  cooling  the  gases  before  they  enter  the  chim- 
ney, reduces  the  average  temperature  of  the  hot 
'gases  while  in  the  chimney  and  thereby  brings 
about  a  reduction  of  the  draft  that  would  other- 
wise be  created  by  a  chimney  of  a  given  height, 
and  necessitates  a  higher  chimney  to  create  a 
given  draft  than  would  be  necessary  if  there  were 
no  economizer.  The  economizer  should  not  be 


GENERAL   DISCUSSION.  IQ 

looked  upon  as  a  necessary  adjunct  to  mechani- 
cal draft,  and  the  cost  of  it  should  not  be  in- 
cluded as  a  part  of  the  cost  of  the  mechanical 
draft  apparatus,  for  while  the  economizer  is  al- 
most always  a  useful  and  economical  adjunct 
to  a  power  plant  its  usefulness  and  economy  do 
not  depend  upon  how  the  necessary  draft  is  ob- 
tained. An  economizer  working  with  the  hot 
gases  entering  it  at  550  degrees  and  the  feed 
water  entering  at  120  degrees  will  bring  about  a 
certain  reduction  of  the  temperature  of  the  gases 
and  a  certain  increase  of  the  temperature  of  the 
feed  water,  but  both  of  these  changes  in  tempera- 
ture will  be  entirely  independent  of  the  method 
adopted  for  creating  the  draft  which  makes  the 
gases  pass  through  the  economizer.  And  while 
an  economizer  is  a  good  thing  it  is  no  more  nec- 
essary with  a  mechanical  draft  plant  than  it  is 
with  a  chimney. 


CHAPTER  II. 

FORCED    DRAFT. 

Systems.  There  are  two  systems  of  installing 
a  forced  draft  apparatus.  The  first  consists  in 
making-  the  boiler-room  or  the  fire-room  air-tight 
and  creating  therein  by  means  of  a  blower  or  fan 
a  pressure  of  air  greater  than  that  of  the  atmos- 
phere ;  the  second  consists  in  making  the  ash-pit 
air-tight  and  creating  therein  a  pressure  of  air 
greater  than  that  of  the  atmosphere.  The  first 
system  is  known  as  the  closed  boiler-room  or 
closed  fire-room  system ;  and  the  second  is  known 
as  the  closed  ash-pit  system.  The  first  system  is 
used  principally  on  ship-board  because  the  air 
supplied  to  the  furnace  for  the  combustion  of  the 
coal  serves  at  the  same  time  to  ventilate  and  aid 
in  keeping  cool  the  fire-room  in  which  the  firemen 
are  obliged  to  work.  It  is  not  used  on  land  be- 
cause of  the  difficulty  and  expense  of  making 
and  keeping  the  boiler-room  or  the  fire-room  air- 
tight. The  second  system  is  used  almost  entirely 
on  land  when  a  forced  draft  is  used  at  all,  al- 
20 


FORCED    DRAFT.          -  21 

though  it  has  several  disadvantages  as  compared 
to  the  first  system. 

^^losed  Fire-Room  System.  With  the  first  or 
closed  fire-room  system  the  pressure  of  the  air 
on  the  outside  of  the  ash-pit  and  the  furnace  is 
greater  than  on  the  inside,  and,  therefore,  there 
is  a  leakage  of  air  into  the  furnace  and  ash-pit 
instead  of  a  leakage  of  hot  gases  out  into  the 
boiler-room,  as  there  is  when  the  second  system 
is  used.  Again,  there  is  no  blowing  out  of  hot 
gases  or  cinders  into  the  boiler-room  when  the 
door  is  opened  to  put  in  a  fresh  charge  of  fuel 
with  this  system  as  there  is  apt  to  be  with  the 
closed  ash-pit  system,  unless  the  pressure  main- 
tained in  the  ash-pit  is  no  more  than  just  suffi- 
cient to  overcome  the  resistance  to  the  flow  of 
the  air  through  the  bed  of  fuel  on  the  grate. 
Further,  because  the  leakage  in  the  case  of  this 
system  is  inward  instead  of  outward  as  in  the 
other,  the  boiler  setting  is  not  made  so  hot,  and, 
therefore,  does  not  deteriorate  so  rapidly  when 
the  first  system  is  used  as  when  the  second  sys- 
tem is  used.  /This  last  advantage  of  this  system 
over  the  closed  ash-pit  system  does  not  apply  to 
the  case  of  internally  fired  boilers,  but  only  to 
externally  fired  boilers  with  brick  settings. 


22  MKCIIANICAL    DRAFT. 

Closed  Ash-Pit  System.  In  order  to  avoid 
the  objectionable  features  of  forced  draft  as  used 
on  land  with  a  pressure  in  the  ash-pit  and  not  in 
the  boiler-room,  it  is  usual  to  create  in  the  ash-pit 
a  pressure  sufficient  to  overcome  only  the  resist- 
ance to  the  passage  of  the  air  through  the  fire 
on  the  grate,  and  no  more.  That  is,  the  pressure 
in  the  ash-pit  is  slightly  greater  than  that  of  the 
atmosphere,  while  in  the  furnace  it  is  equal  to  or 
slightly  less  than  that  of  the  atmosphere.  The 
result  is  that  a  chimney  must  be  provided  of  a 
height  sufficient  to  create  a  draft  to  make  the 
gases  flow  from  the  furnace  through  the  flues  and 
passages  into  the  chimney  and  then  out  into  the 
atmosphere.  As  the  resistance  to  the  flow  of  the 
gases  through  the  fire  is  about  one-half  the  total 
draft  required  when  there  is  no  economizer,  the 
draft  which  must  be  created  by  the  fan  or  blower 
is  about  one-half  the  total  draft  which  would  be 
necessary  for  the  complete  combustion  of  the  fuel. 
And  since  the  draft  produced  by  a  chimney  is 
directly  proportional  to  its  height,  it  follows  that 
where  a  forced  draft  apparatus  is  used  to  pro- 
duce a  pressure  in  the  ash-pit  sufficient  to  over- 
come only  the  resistance  to  the  flow  of  the  air 
through  the  fuel  on  the  grate,  there  must  be  a 
chimney  whose  height  is  about  one-half  the  height 


FORCED   DRAFT.  2$ 

of  the  chimney  which  would  be  required  for  the 
combustion  of  the  same  quantity  of  fuel  without^^ 
any  forced  draft  apparatus.  Or  to  put  it  in 
another  way,  by  the  addition  of  a  forced  draft 
apparatus  for  producing  a  pressure  in  the  ash- 
pit, the  draft  of  a  power  plant  can  be  doubled 
without  any  trouble. 

When  the  pressure  produced  in  the  ash-pit  is 
greater  than  that  necessary  to  overcome  the  fric- 
tion of  the  air  through  the  fuel  on  the  grate,  there 
is  a  pressure  in  the  furnace ;  and  it  then  becomes 
necessary  to  shut  off  entirely  or  at  least  reduce 
the  pressure  produced  by  the  apparatus,  when 
the  fireman  desires  to  open  the  furnace  door  to 
fire  or  tend  the  furnace.  If  this  is  not  done  hot 
gases  and  even  cinders  are  blown  out  into  the 
boiler-room,  to  the  great  discomfort  of  the  fire- 
men. For  the  same  reason  it  is  always  necessary 
to  shut  off  the  draft  before  opening  the  ash-pit 
door  to  clean  out  the  ash-pit. 

The  closed  ash-pit  system  of  applying  mechan- 
ical draft  is  absolutely  necessary  with  certain 
forms  of  furnaces  and  mechanical  stokers  which 
are  so  constructed  as  to  make  the  friction  of  the 
air  through  the  fuel  very  great. 

Small  Fan  Required.     Because  the  air  handled 


24  MECHANICAL   DRAFT. 

by  a  forced  draft  apparatus  is  at  a  low  tempera- 
ture, never  exceeding  the  temperature  of  the 
boiler-room,  the  volume  of  air  is  not  so  great  as 
it  would  be  if  handled  at  the  temperature  of  the 
gases  in  the  chimney  or  the  temperature  of  the 
gases  handled  by  an  induced  mechanical  draft 
fan.  This  means,  of  course,  that  the  fan  required 
for  a  forced  draft  apparatus  may  be  smaller  than 
would  be  required  for  an  induced  draft  apparatus 
for  the  combustion  of  the  same  quantity  of  fuel; 
and  it  also  means  that  since  the  air  handled  by 
the  fan  is  comparatively  cool,  the  bearings  are 
easier  to  keep  cool,  and  no  water  is  required  for 
cooling  them,  as  is  always  required  in  the  case 
of  an  induced  draft  apparatus. 

Usual  Pressure.  It  is  a  curious  thing  that 
while  a  forced  draft  apparatus  by  which  a  pres- 
sure is  maintained  in  the  ash-pit  just  sufficient  to 
overcome  the  resistance  to  the  passage  of  the  air 
through  the  grate,  need  not  be  capable  of  main- 
taining a  pressure  in  the  ash-pit  of  much  if  any 
more  than  0.6  of  the  total  draft  required  for  the 
combustion  of  the  fuel,  the  fan  used  is  almost 
always  put  in  to  handle  the  air  at,  and  capable 
of  creating  in  the  ash-pit,  a  much  higher  draft 
than  required,  higher  even  than  an  induced  me- 


K 

W 


FORCED    DRAFT.  2$ 

chanical  draft  would  be  designed  for,  and  the 
proper  pressure  in  the  ash-pit  is  secured  by  throt- 
tling the  pressure  created  by  the  fan.  This  is 
done  in  order  to  reduce  the  first  cost  of  the  appa- 
ratus, so  that  the  total  cost  of  the  fan  and  its 
motor,  together  with  the  cost  of  the  chimney 
which  must  be  used  in  connection  with  it  may  be 
kept  as  low  -s  possible.  This  procedure,  while 
reducing  the  -Irst  cost  of  the  mechanical  draft 
apparatus,  increases  the  cost  of  operation,  as  it 
requires  more  power  to  run  the  fan  so  as  to  give 
the  higher  pressure  than  would  be  required  to 
run  it  to  give  the  lower  pressure  which  is  neces- 
sary. The  fans  are  usually  chosen  of  such  a 
size  that  they  must  be  run  at  a  speed  sufficient 
to  give  a  pressure  of  il/2  or  2  Miches  of  water 
in  order  to  handle  the  volume  of  air  required 
for  the  combustion  of  the  fuel. 

Thus,  suppose  a  certain  plant  is  to  be  designed 
to  burn  5,000  pounds  of  coal  per  hour  with  a 
draft  of  one  inch.  A  forced  draft  apparatus 
would  consist  of  a  fan  capable  of  giving  a  pres- 
sure in  the  ash-pit  of  about  0.6  of  an  inch  and 
a  chimney  at  least  80  feet  high.  A  fan  running 
at  a  speed  to  give  only  the  required  pressure  of 
0.6  of  an  inch  would  require  a  wheel  6^/2  feet 
in  diameter  to  handle  the  air  required,  while  a 


26  MECHANICAL    DRAFT. 

fan  running  at  a  speed  to  give  a  pressure  of 
about  1.5  inches  while  handling  the  required 
amount  of  air  would  have  a  wheel  only  5  feet  in 
diameter.  It  is  probable  that  the  smaller  wheel 
would  be  used,  although  the  power  required  to 
run  it  would  be  2^/2  times  what  would  be  required 
to  run  the  larger  fan,  and  the  pressure  in  the 
ash-pit  would  be  regulated  by  throttling  the  air 
at  the  point  of  delivery  into  the  ash-pit. 

Forced  Draft  and  Economizers.  When  an 
economizer  is  used  with  the  closed  ash-pit  system 
of  forced  draft,  the  chimney  must  be  of  sufficient 
height  to  overcome  the  friction  of  the  gases 
through  the  economizer  as  well  as  through  the 
flues  of  the  boiler  and  through  the  chimney  itself, 
as  the  pressure  due  to  the  fan  is  not  supposed  to 
extend  beyond  the  furnace. 

Advantages.  The  advantages  claimed  for  the 
closed  ash-pit  system  of  forced  draft  are : 

1  —  Small  volume  of  air  to  be  handled.     This 
is  due  to  the  low  temperature  at  which  the  air 
is  handled. 

2  —  Small  first  cost  of  mechanical  apparatus. 
The  cost  meant  here  does  not  include  the  cost 
of  the  chimney,  which  is  absolutely  necessary  in 


FORCED  DRAFT.  2.J 

conjunction  with  this  system  of  mechanical  draft. 
And,  further,  the  cost  of  the  mechanical  appa- 
ratus is  usually  reduced  by  putting  in  a  small 
apparatus  and  increasing  the  running  expense. 

3  —  No  danger  of  overheating  of  the  bearings, 
and  no  water  required  for  cooling  them.  This  is 
due  to  the  low  temperature  of  the  air  handled 
by  the  apparatus. 

Disadvantages.  The  disadvantages  of  the 
closed  ash-pit  system  are : 

1  --The  necessity  of  a  chimney  of  a  height  to 
give  a  draft  sufficient  to  overcome  all  the  resist- 
ances to  the  flow  of  the  gases  through  the  differ- 
ent flues  and  out  of  the  chimney ;  that  is,  all  the 
draft  except  what  is  necessary  to  overcome  the 
resistance  due  to  the  fuel  on  the  grate. 

2  —  Less  flexibility  and  less  control  of  draft. 
This  is  due  to  the  fact  that  part  of  the  draft, 
about  40  or  50  per  cent,  is  natural  or  chimney 
draft,  the  remainder  only  being  mechanical  draft. 

3  —  Not    adapted    for   use    with    economizers. 
This  is  not  a  valid  objection  if  the  chimney  be 
made  high  enough  to  give  a  draft  sufficient  to 
overcome  the  resistance   due  to  the   economizer 
as  well  as  that  due  to  the  friction  of  the  gases  in 
their  passage  from  the  furnace  to  the  top  of  the 


28  MECHANICAL    DRAFT. 

chimney.  If  the  chimney  is  not  high  enough  to 
give  this  required  draft  the  objection  is  valid, 
as  the  pressure  due  to  the  fan  is  not  supposed  to 
be  more  than  sufficient  to  overcome  the  friction 
of  the  air  through  the  fire  on  the  grate. 

4  —  Leakage  of  air  outward  into  the  boiler- 
room.     With  a  good  ash-pit  this  leakage  is  not 
great,  and  the  objection  cannot  amount  to  much 
so  long  as  the  pressure  is  not  so  great  as  to  make 
a  pressure  in  the  furnace.     If  the  pressure  is  so 
great  as  to  make  a  pressure  in  the  furnace  then 
there  is  a  leakage  of  hot  gases  outward,  which 
is  bad  for  the  firemen  and  causes  an  overheating 
of  the  boiler  setting.     The  main  objection  to  the 
outward  leakage  is  that  it  requires  that  the  draft 
shall    be    shut   off    when    the   ash-pit    doors    are 
opened  to  clean  the  ash-pit,  as  if  this  is  not  done 
ashes  will  be  blown  into  the  boiler-room. 

5  —  Difficulty  of  controlling  the  fire.     Because 
of  the  fact  that  the  ash-pit  doors  must  be  kept 
closed  it  is  more  difficult  to  keep  watch  of  the 
fire  when  the  closed  ash-pit  system  of  mechani- 
cal draft  is  used  than  when  chimney  or  induced 
mechanical  draft  is  used.     And  unless  the  air  is 
admitted  so  as  to  be  uniformly  distributed  over 
the   underside   of   the   grate,   the   combustion   is 
likely  to  be   very   much   more   rapid   in   certain 


FORCED    DRAFT.  29 

places  than  in  others,  so  that  there  is  a  tendency 
for  the  fire  to  burn  out  in  spots.  When  this 
happens  the  air  rushes  through  these  spots  into 
the  furnace,  and  by  impinging  on  certain  parts 
of  the  boiler,  because  of  its  temperature  being 
lower  than  that  of  the  products  of  combustion, 
causes  a  partial  cooling  of  these  parts,  and  there- 
fore, causes  an  unequal  expansion  in  the  parts  of 
the  boiler  which  is  likely  to  result  in  trouble  of 
some  kind  or  other.  This  is  much  more  likely 
to  occur  with  a  high  pressure  than  a  low  pres- 
sure. In  fact,  when  the  outlet  for  the  air  into 
the  ash-pit  is  properly  designed  and  located,  and 
the  pressure  maintained  in  the  ash-pit  is  not 
greater  than  that  necessary  to  overcome  the  re- 
sistance due  to  the  fire  on  the  grate,  there  is  little 
trouble  from  burning  through  of  the  fire  in  spots, 
and  this  disadvantage  is  not  often  apparent  in 
the  case  of  the  closed  ash-pit  system  as  applied 
to  boilers  on  land. 


CHAPTER  III. 

INDUCED  DRAFT. 

Introduction.  The  fan  used  in  an  induced 
draft  system  is  placed  beyond  the  boiler  in  the 
uptake  and  by  its  action  a  partial  vacuum  is 
maintained  in  the  uptake,  the  flues  and  other  pas- 
sages ^for  the  gases,  and  in  the  furnace.  The  air 
enters  the  furnace  because  the  pressure  there  is 
less  than  that  of  the  atmosphere.  The  gases  of 
combustion  flow  from  the  furnace  to  the  fan, 
and  then  are  discharged  into  the  atmosphere 
through  a  chimney  which  need  be  no  higher  than 
absolutely  necessary  to  make  the  gases  clear  the 
neighboring  buildings,  so  that  usually  the  chim- 
ney is  short  and  it  may  be  of  comparatively  cheap 
construction. 

As  all  the  draft  is  due  to  the  action  of  the  fan, 
it  is  under  perfect  control  and  may  be  increased 
or  decreased  at  will  by  speeding  up  or  slowing 
down  the  fan.  The  draft  produced  is  of  exactly 
the  same  kind  as  that  produced  by  a  chimney ; 
and  since  the  pressure  inside  of  the  boiler  setting 
30 


INDUCED   DRAFT.  3! 

and  furnace  is  less  than  that  of  the  atmosphere 
the  leakage  is  from  without  in,  so  that  there  is 
no  escape  of  hot  gas  into  the  boiler-room,  nor 
any  blowing  out  'of  ashes  or  cinders  into  the 
boiler-room  when  the  furnace  or  ash-pit  doors 
are  opened.  Further,  there  is  no  tendency  to 
make  the  walls  of  the  boiler  setting  hotter  by 
forcing  hot  air  into  them.  Neither  is  there  the 
same  tendency  for  the  fire  to  burn  through  in 
spots  that  there  is  with  the  closed  ash-pit  system 
of  forced  draft,  because  there  is  no  jet  of  air 
impinging  on  any  one  spot  of  the  fire  with  more 
force  than  on  others.  The  flow  of  air  from  the 
ash-pit  into  the  furnace  is  uniform  and  regular 
over  all  parts  of  the  grate  which  are  covered  to 
the  same  thickness  with  fuel. 

The  fan  may  be  speeded  up  or  slowed  down 
automatically  to  increase  or  decrease  the  combus- 
tion in  order  to  preserve  a  uniform  pressure  in 
the  boiler,  or  it  may  be  run  'at  a  speed  sufficient 
to  give  the  maximum  draft  required  and  then 
the  draft  necessary  may  be  obtained  by  changing 
the  opening  of  a  damper  placed  in  the  uptake 
between  the  fan  and  the  boiler,  in  the  same  man- 
ner in  which  the  draft  is  regulated  when  pro- 
duced by  a  chimney.  Since  the  capacity  of  a  fan, 
that  is  to  say,  the  number  of  cubic  feet  of  gas 


32  MECHANICAL    DRAFT. 

that  it  can  handle  per  minute,  varies  with  the 
speed  at  which  it  is  run,  the  amount  of  coal 
burned  per  minute  can  be  increased  by  simply 
increasing  the  speed  of  the  fan.  And  as  the 
capacity  of  the  plant  to  create  steam  varies  al- 
most directly  as  the  amount  of  coal  burned  per 
hour,  it  is  a  very  easy  matter  to  increase  the 
capacity  of  the  plant  as  it  at  most  means  putting 
in  a  larger  engine  to  do  the  work  necessary  .to 
run  the  fan  faster. 

Temperature  of  the  Gases.  Inasmuch  as  the 
fan  of  an  induced  mechanical  draft  apparatus 
handles  the  hot  gases  instead  of  the  cool  air  for 
combustion,  the  fan  acquires  a  temperature  al- 
most equal  to  that  of  the  gases,  and  as  this  tem- 
perature may  be  as  high  as  600  degrees  or  even 
higher  it  is  necessary  to  use  water  cooled  bear- 
ings for  it.  Further,  as  the  volume  of  the  gases 
"increases  with  the  temperature,  the  higher  the 
temperature  the  greater  is  the  volume  of  gases 
which  the  fan  must  handle ;  and  for  a  fan  with 
a  wheel  of  a  given  diameter  this  means  a  higher 
speed  of  rotation  and  more  power  to  drive  the 
fan.  The  volume  of  the  gases  resulting  from 
f  the  combustion  of  a  given  quantity  of  coal  when 
heated  to  a  temperature  of  about  550  degrees, 


INDUCED   DRAFT.  33 

quite  a  common  temperature  for  chimney  gases,  1 
is  about  twice  as  great  as  the  volume  of  the  air  / 
supplied  to  the  furnace  when  at  a  temperature^ 
of  about  80  degrees.     Whatever,  then,  reduces 
the  temperature  of  the  gases  before  they  enter 
the   fan,  reduces  the  volume  per  pound  of  the 
gases  and  therefore  increases  the  weight  of  gases 
which  the  fan  can  handle  when  running  at  a  given 
speed,  because  the  capacity  of  a  fan  like  that  of  a 
pump  depends  upon  its  volumetric  displacement 
per   unit  of  time.     The  greater  the   weight  of 
gases    handled    per    minute    the    greater    is    the 
weight  of  coal  burned  per  hour  and  hence  the 
greater  the  capacity  of  the  boiler  plant  as  a  stearn 
producer.     In    other    words,    cooling    the    gasesj) 
before  they  reach  the  fan  not  only  increases  the 
draft  for  a  given  speed  of  the  fan,  but  also  in- 
creases the  amount  of  coal  which  may  be  burned 
per  hour  and  hence  increases  the  capacity  of  the    \ 
plant  as  a  steam  producer,  and  does  not  materially   , 
change  the  power  required  to  run  the  fan.     And 
if  the  gases  can  be  cooled  by  increasing  the  heat- 
ing surface  of  the  boilers  or  by  the  use  of  an 
economizer  or  an  air  heater,  the  heat  taken  from 
the  gases   during  the   cooling  process   is   saved 
and  the  steam  generated  by  the  plant  is  obtained 
more  economically. 


34  MECHANICAL   DRAFT. 

With  a  chimney  the  case  is  very  different,  since 
a  reduction  in  the  temperature  of  the  gases  en- 
tering the  chimney  means  a  reduction  in  the 
draft  as  well  as  a  reduction  in  the  volume  of 
gases  flowing  out  of  the  chimney.  If  the  reduc- 
tion in  the  temperature  be  great,  the  draft  may 
not  be  great  enough  to  overcome  the  friction  of 
the  air  through  the  fire  and  the  friction  of  the 
hot  gases  through  the  various  flues  and  the  chim- 
ney. The  result  is  that  because  of  the  reduction 
of  the  draft  and  in  spite  of  the  less  volume  of 
gases  to  be  handled  by  the  chimney,  the  amount 
of  coal  that  can  be  burned  in  the  furnace  may 
be,  and  is  very  likely  to  be  unless  the  draft  in  the 
first  place  was  greater  than  necessary,  less  after 
the  gases  are  cooled  than  before.  This  means, 
course,  a  decrease  in  the  capacity  of  the  whole 
plant  as  a  steam  producer,  and  may  necessitate 
an  increase  in  the  whole  plant  in  order  that  it 
may  generate  the  required  amount  of  steam  per 
hour.  And  the  interest  on  the  first  cost  and  the 
depreciation  and  repairs  on  the  addition  to  the 
plant  may  more  than  offset  the  saving  made  by 
cooling  the  gases.  Of  course,  when  the  chimney 
is  high  enough  to  give  the  required  draft  with 
the  lower  temperature  of  the  gases,  cooling  them 
brings  about  a  saving  in  the  operation  of  the 


FIG.  4.— INDUCED  DRAFT  APPARATUS,  VICTORIA  HOTEL, 

NEW  YORK. 
(American  Blower  Co.) 


INDUCED   DRAFT.  35 

plant  by  reducing  the  quantity  of  coal  necessary 
to  be  burned  in  order  to  evaporate  the  required 
amount  of  water. 

Advantages.     The  advantages  claimed  for  the 
induced  system  of  mechanical  draft  are: 

1  —  Low  first  cost.     For  isolated  plants  where 
the  nearness  to  neighboring  buildings  does  not 
make  it  necessary  to  erect  a  tall  chimney  in  order 
to  discharge  the  gases  at  a  great  height  above 
the  ground,  the  cost  of  an  induced  draft  system 
will  always  be  less  than  the  cost  of  a  substantial 
chimney  unless  the  plant  be  small.     And  usually, 
the  cost  will  be  less  than  the  cost  of  the  mechan- 
ical equipment  and  chimney  necessary  for  a  sys- 
tem of  closed  ash-pit,  forced  draft. 

2  —  No  necessity  for  a  chimney.     An  induced 
draft  plant  never  needs  a  chimney  to  aid  in  pro- 
ducing the    draft,    and    whenever   a   chimney   is 
used  with  one  it  is  made  necessary  by  other  con- 
siderations than  the  draft.     When  anything  more 
than   a  short  stack  is  required,  the  cost  of  the 
additional  chimney  should  not  be  charged  as  a 
disadvantage  against  the  mechanical  draft  plant 
although  it  must,  of  course,  be  included  in  the 
total  cost  of  the  draft  producing  apparatus  and 
may  often  make  the  decision  of  how  to  produce 


36  MECHANICAL   DRAFT. 

the  required  draft  adverse  to  the  induced  system. 

3  —  Control  of  draft.     Since  the  draft  is  due 
entirely  to  the  action  of  the  fan  and  increases  and 
decreases  as  the  fan  is  run  faster  or  slower,  it  is 
evident  that  it  is  entirely  under  control  and  can 
be  varied  at  will  to  suit  the  requirements  of  the 
instant.    With  a  chimney  a  hot  fire  means  a  high 
temperature  of  the  escaping  gases  and  therefore 
a  high  draft,  and  a  low  fire  means  a  low  draft; 
while  with  an  induced  draft  system  the  draft  may 
be  made  low  with  a  hot  fire  or  high  with  a  low 
fire,  and  is  always  independent  of  everything  ex- 
cept the  speed  of  the  fan. 

4  —  Uniform  combustion.     The  combustion  in 
a  furnace  equipped  with  an  induced  system  of 
mechanical    draft   is    just   as   uniform   over   the 
whole  surface  of  the  grate  as  it  is  with  ordinary 
chimney  draft.     There  is  no  burning  through  of 
the  fire  in  spots  as  there  is  likely  to  be  with  the 
closed  ash-pit  system  of  forced  draft,  especially 
with  high  drafts. 

5  —  Leakage  inward.    This  makes  it  easier  for 
the  fireman  to  tend  to  the  furnace  and  ash-pit, 
avoids  the  trouble  due  to  blowing  hot  gases  or 
air,  or  cinders  or  ashes  into  the  fire-room,  even 
when  the  furnace  or  ash-pit  doors  are  open ;  and 
further,  there  is  not  the  same  tendency  to  increase 


INDUCED    DRAFT.  37 

the  deterioration  of  the  boiler  or  its  setting  that 
there  is  with  the  closed  ash-pit  system  of  forced 
draft.  The  leakage  is  exactly  the  same  as  with 
ordinary  chimney  draft  and  produces  no  more 
bad  effects  for  the  same  intensity  of  draft. 

6  —  Adaptability  to  use  with  economizers. 
This  is  one  of  the  strong  points  in  favor  of  the 
induced  draft  system,  as  cooling  the  gases  after 
they  leave  the  boiler  results  in  an  increase  in  the 
number  of  pounds  of  water  evaporated  per  pound 
of  coal,  and,  therefore,  an  economy  in  the  opera- 
tion of  the  boiler  plant  without  affecting  the 
draft.  It  is  possible  to  do  any  one  of  three  things 
when  an  economizer  is  used  in  connection  with 
an  inducecl  draft  system. 

(a)  Reduce  the  speed  of  the  fan  until  it  gives 
only  the   draft   necessary   to   burn   the   required 
amount  of  coal.     This  results  in  using  less  steam 
to  run  the  fan  and  this  saving  in  steam,  together 
with  the  saving  due  entirely  to  the  economizer, 
will  usually  result  in  a  marked  economy  in  the 
operation  of  the  plant. 

(b)  Run  the  fan  at  the  same  speed  and  burn 
a  larger  quantity  of  coal  of  a  cheaper  grade,  so 
as  to  evaporate  the  same  amount  of  water  per 
hour  with  the   economizer  that   was  evaporated 
before.     This    almost    invariably    results    in    a 


3&'  MECHANICAL    DRAFT. 

marked  reduction  in  the  coal  bill,  and,  therefore, 
since  other  things  remain  the  same,  in  an  econ- 
omy in  the  operation  of  the  plant. 

(c)  Run  the  fan  at  the  same  speed  but  burn  a 
larger  quantity  of  the  same  coal  than  was  used 
before  the  introduction  of  the  economizer.  This 
results  in  an  increase  in  the  coal  bill,  with  an 
increase  in  the  steam  capacity  of  the  plant  while 
keeping  the  fixed  charges,  such  as  wages  and 
cost  of  operation  of  the  fan,  the  same.  The  in- 
crease in  the  capacity  is  always  greater  than  the 
increase  in  the  coal  bill ;  and  since  the  fixed 
charges  against  the  plant  are  increased  only  by 
the  interest  on  the  first  cost,  depreciation,  repairs 
and  operating  expenses  of  the  economizers,  the 
cost  of  making  a  pound  of  steam  is  materially 
reduced. 

Disadvantage^.  The  disadvantages  urged 
against  the  use  of  the  induced  system  of  me- 
chanical draft  are : 

i  —  High  temperature  at  which  the  fans  must 
operate.  This  resolves  itself  into  simply  a  ques- 
tion of  fact  whether  the  fans  can  be  successfully 
operated  at  the  high  temperature  usually  found 
in  chimneys  either  with  or  without  economizers, 
and  experience  has  proved  beyond  a  shadow  of 


INDUCED    DRAFT.  ""*  39 

a  doubt  that  it  is  perfectly  feasible  to  operate 
fans  even  when  th£  gases  enter  them  at  a  tem- 
perature as  high  as  600  degrees,  the  temperature 
of  melting  lead.  This,  however,  is  one  of  the 
objections  to  the  use  of  induced  draft  systems 
on  war  vessels,  where  at  times  the  rate  of  com- 
bustion may  be  such  that  the  temperature  of  the 
escaping  gases  may  be  nearer  1,000  than  600 
degrees. 

2  —  Expense     of     operating.     The     operating 
expenses    of    induced    draft    systems    when    the 
gases   leave  at  the   same  temperature   at   which 
they   should  leave  a  chimney,   are  greater  than 
the  operating  expenses  of  a  chimney  by  the  cost 
of  attendance,  steam  used  for  running  the  fan, 
oil  used  for  lubrication,  and  water  used  for  cool- 
ing the  bearings ;  and  in  such  a  case  if  the  cost  of 
operating  is  not  more  than  offset  by  interest  on 
first  cost,  depreciation  and  repairs,  and  economy 
in  the  operation  of  the  plant  as  a  whole,  it  is 
evident  that  the  chimney  is  the  better  method  of 
producing  the  required  draft. 

3  —  Water  required  to  cool  the  bearings.    Or- 
dinarily   there   is    no   trouble   to    get    the    small 
amount  of  water  required  to  cool  the  bearings 
of  the  fan,  although  if  this  water  be  wasted  it 
adds    to    the    cost    of    operating   the    apparatus. 


4O  MECHANICAL   DRAFT. 

When  water  under  pressure  is  available  the  cost 
is  simply  that  of  the  water,  but  when  the  water 
must  be  pumped  by  means  of  some  form  of  cir- 
culating pump  to  make  it  circulate  from  the 
source  of  supply  through  the  bearings  to  be 
cooled,  the  cost  of  running  the  circulating  pump, 
must  be  added  to  the  cost  of  the  water  itself  if 
there  be  any. 


CHAPTER  IV. 

FUEL    AND    AIR. 

Weight  of  Coal  to  be  Burned.  Since  the  ob- 
ject of  a  draft  is  to  supply  air  for  the  combustion 
of  a  quantity  of  coal  per  hour,  the  first  thing  to 
be  done  in  designing  an  apparatus  for  producing 
the  draft  of  a  power  plant  is  to  determine  the 
weight  of  coal  which  must  be  burned  per  hour 
under  the  given  conditions.  The  weight  of  coal 
required  to  be  burned  per  hour  depends  upon  the 
weight  of  water  which  must  be  evaporated  per 
hour  from  and  at  212  degrees,  and  the  number 
of  pounds  of  water,  from  and  at  212  degrees, 
that  can  be  evaporated  per  pound  of  coal  burned, 
and  is  always  equal  to  the  first  of  these  two 
quantities  divided  by  the  second.  That  is,  if  W 
be  the  weight  of  water  from  and  at  212  degrees 
to  be  evaporated  per  hour,  and  iv  be  the  weight 
of  water  from  and  at  212  degrees  which  can  be 
evaporated  per  pound  of  coal,  the  weight  of  coal, 
C  ,  which  it  is  necessary  to  burn  per  hour  is 


41 


42  MECHANICAL   DRAFT. 

W  depends  upon  the  size  and  type  of  the  en- 
gines used ;  the  conditions  under  which  they  are 
used,  condensing  or  non-condensing ;  the  steam 
used  by  feed  pumps  and  other  auxiliary  appara- 
tus ;  and  the  steam  used  for  heating  or  other  pur- 
poses. It  must  be  determined  for  each  plant,  as 
each  becomes  a  distinct  and  separate  problem 
because  of  its  own  peculiar  conditions  of  woiking. 

Evaporation  per  Pound  of  Coal.  The  num- 
ber of  pounds,  w,  of  water,  from  and  at  212 
degrees,  that  can  be  evaporated  per  pound  of 
coal  depends  upon  the  heating  power  of  the  coal 
used ;  the  completeness  of  combustion  in  the  fur- 
nace ;  the  rate  of  evaporation  per  square  foot  of 
heating  surface  of  the  boiler ;  and  the  type  of 
boiler  or  kind  of  heating  surface.  While  there 
are,  of  course,  other  things  which  may  affect  the 
value  of  w,  those  mentioned  are  the  main  ones. 
For  most  boiler  plants  there  is  little  or  no  trouble 
in  having  the  combustion  practically  complete, 
provided  a  sufficient  quantity  of  air  is  admitted 
to  the  furnace. 

For  boilers  of  the  return  fire  tube  type  with 
good  brick  settings,  the  maximum  number  of 
pounds  of  water  is  found  to  be  evaporated  per 
pound  of  coal  when  the  rate  of  evaporation  per 


FUEL   AND   AIR.  43 

square  foot  of  heating  surface  is  about  2.8 
pounds  of  water,  from  and  at  212  degrees,  per 
hour ;  and  for  boilers  of  the  water  tube  type  the 
maximum  evaporation  per  pound  of  coal  is 
reached  when  the  rate  of  evaporation  per  square 
foot  of  heating  surface  is  about  3.3  pounds  of 
water,  from  and  at  212  degrees,  per  hour.  That 
is  to  say,  the  maximum  value  of  w  is  reached 
when  the  rate  of  evaporation  per  square  foot  of 
heating  surface  per  hour,  in  pounds  of  water  from 
and  at  212  degrees,  is  about  2.8  pounds  for  fire 
tube  boilers,  and  3.3  for  water  tube  boilers. 

A  study  of  the  tests  of  boilers  made  under  dif- 
ferent conditions  seems  to  indicate  that  for  ordi- 
nary draft  and  hand  firing  without  an  economizer, 
the  maximum  value  of  w  varies  for  different  coals 
in  such  a  way  that  if  H  is  the  heating  power 
of  the  coal,  the  probable  maximum  value  of  w  is 

H 

(2)         w  = 3 

1000 

Table  I  shows  the  probable  maximum  values 
of  w  as  calculated  by  equation  (2)  for  coals  hav- 
ing different  values  of  H. 

Effect  of  Rate  of  Evaporation.  How  much 
w  may  fall  below  the  probable  maximum  value 


44 


MECHANICAL    DRAFT. 


as  given  by  Table  I  will  depend  principally  upon 
the  rate  of  evaporation  per  hour  per  square  foot 
of  heating  surface  of  the  boiler,  and  must  be  ob- 

TABLE  I. 
Maximum  Evaporation  for  Different  Coals. 


1 

Heating 
power 
of  coal. 
H 

Probable 
maximum 
evaporation 
per  Ib.  of  coal. 

9000 

6 

IOOOO 

7 

1  1000 

8 

12000 

9 

13000 

10 

14000 

ii 

tained  for  each  particular  case.  As  a  guide  the 
following  formula,  in  which  c  is  the  rate  of 
evaporation  per  hour  per  square  foot  of  heating 
surface  in  pounds  of  water  from  and  .at  21 2  de- 
grees, may  be  used : 


(3) 


w  =  z 

2 


z  depends  upon  the  heating  power,  H,  of  the 
coal  and  upon  the  type  of  boiler  or  the  kind  of 


1 

^   2 

a,    G 

-S  8 

*^  w 


_, 

2  § 


O 
O 


FUEL    AND    AIR. 


45 


heating    surface,     and     its     value     is    given     in 
Table  II. 

TABLE  II. 

Values  of  z. 


Heating 

Kind    of    boiler. 

power 

of    coal. 

Return 

Water 

H 

fire  tube. 

tube. 

9000 

7-4 

7-7 

IOOOO 

8-4 

8-7 

IIOOO 

9-4 

9-7 

I2OOO 

10.4 

10.7 

13000 

11.4 

ii.  7 

14000 

12.4 

12.7 

It  must,  of  course,  be  remembered  that  no 
single  formula  can  be  given  which  will  give  ex- 
act results  for  all  conditions  such  as  poor,  leaky 
settings,  bad  firing,  insufficient  air  supply,  and 
the  thousand  and  one  other  conditions  which  may 
decrease  the  evaporating  power  of  a  coal  if  they 
be  allowed  to  exist,  but  which  should  not  exist 
in  a  properly  designed  and  cared  for  boiler  plant. 
And  it  must  further  be  remembered  that  econo- 
mizers and  other  heat-saving  devices  will  effect 
an  increase  in  the  evaporation  per  pound  of  coal 


46  MECHANICAL    DRAFT. 

by  an  amount  which  can  be  predicted  fairly  well 
only  when  the  circumstances  and  conditions  un- 
der which  the  heat-saving  devices  are  to  be  used 
are  known. 

Weight  of  Air  Required.  It  may  be  assumed 
without  any  material  error  that  12  pounds  of  air 
are  required  to  supply  the  amount  of  oxygen 
theoretically  necessary  for  the  complete  combus- 
tion of  one  pound  of  coal.  This  would  be  more 
nearly  true  if  the  coal  consisted  of  pure  carbon 
and  contained  no  hydrogen  or  no  incombustible 
matter.  As,  however,  it  is  practically  impossible 
to  be  sure  that  in  the  furnace  each  particular 
atom  of  combustible  will  be  brought  in  direct 
contact  with  the  required  oxygen  if  there  be  only 
the  amount  that  is  theoretically  necessary,  it  is 
necessary  to  introduce'  more  air  than  is  theoret- 
ically necessary  for  the  complete  combustion  of 
the  coal.  And  the  results  of  tests  during  which 
the  per  cents  of  free  oxygen  and  carbon  dioxide 
in  the  gases  of  combustion  have  been  determined, 
show  that  combustion  is  almost  always  complete 
when  the  per  cent  of  oxygen  is  6  or  7  and  the 
per  cent  of  carbon  dioxide  is  14  or  13,  indicating 
about  50  per  cent  more  air  than  is  theoretically 
necessary.  It  may  be  assumed  therefore  that  18 


FUEL    AND    AIR.  47 

pounds  of  air  must  be  supplied  to  the  furnace  for 
each  pound  of  coal  burned. 

Volume  of  Air  and  Gases.  The  volume  of 
one  pound  of  air  measured  at  32  degrees  is  about 
i2l/2  cubic  feet,  and  hence  the  volume  of  air 
measured  at  32  degrees  which  must  be  admitted 
to  the  furnace  for  each  pound  of  coal  burned  is 
18  x  i2l/2,  or  225  cubic  feet.  If  measured  at  a 

temperature  t,  the  volume  would  be  - 

and  this  multiplied  by  the  weight  of  coal  burned 
per  hour  and  divided  by  60  will  be  the  cubic  feet 
of  air  which  the  draft  producing  apparatus  must 
handle  per  minute. 

The  volume  of  the  gases  resulting  from  the 
combustion  in  the  furnace  may  for  all  practical 
purposes  be  considered  as  equal  to  the  volume  of 
the  air  supplied  for  combustion,  when  both  are 
measured  at  the  same  temperature.  Hence  the 
volume  of  the  air  or  the  gases  to  be  handled  per 
minute  by  the  draft  producing  fans  is 


A    - 


493X6o 
+  46i) 
13* 


48 


MECHANICAL    DRAFT. 


A  is  the  volume  in  cubic  feet  of  air  or  gases 
to  be  handled  per  minute ;  C,  the  weight  of  coal 
to  be  burned  per  hour ;  and  t,  the  temperature  at 
which  the  air  or  gases  are  handled  by  the  draft 
producing  fan. 

The  value    of  the    volume  factor  — ,  for 

different  values  of  t  at  which  the  fan  of  a  me- 
chanical draft  apparatus  must  handle  the  air  or 
gases  of  combustion  are  given  in  Table  III. 

TABLE  III. 

Value  of  Volume  Factor. 


Temp,  of  the 

£+46  1 

air  or  gases. 
/ 

131 

60 

4.0 

80 

4.1 

100 

4-3 

200 

5-0 

300 

5.8 

400 

6.6 

500 

7-3 

600 

8.1 

Volume  of  Gases  to  be  Handled.  The  fan 
of  a  forced  draft  apparatus  handles  the  air  re- 
quired for  combustion  at  probably  some  tem- 


FUEL    AND    AIR.  49 

perature  m  the  neighborhood  of  60  degrees ;  and 
as  Table  III  shows  that  the  value  of  ^^  for 
60  degrees  is  4,  we  have  from  equation  (4)  : 
the  number  of  cubic  feet  of  air  to  be  supplied 
per  minute  by  the  fan  of  a  forced  draft  apparatus 
is  equal  to  four  times  the  number  of  pounds  of 
coal  burned  per  hour. 

The  fan  of  an  induced  draft  apparatus  with 
an  economizer  handles  the  gases  of  combustion 
at  a  temperature  usually  between  250  and  350? 
degrees,  so  that  we  may  say  from  Table  III  and-. 
"  equation  (4)  :  the  number  of  cubic  feet  of  gases' 
to  be  handled  per  minute  by  the  fan  of  an  in- 
duced draft  apparatus  zvith  an  economizer  is  equal 
to  six  times  the  number  of  pounds  of  coal  burned 
per  hour. 

The  fan  of  an  induced  draft  apparatus  without 
an  economizer  is  not  likely  to  be  called  upon  to 
handle  the  gases  at  a  temperature  exceeding  600 
degrees  and  hence  we  may  say :  the  number  of 
cubic  feet  of  gases  to  be  handled  per  minute  by 
the  fan  of  an  induced  draft  apparatus  without  an 
economizer  is  equal  to  eight  times  the  number  of 
pounds  of  coal  to  be  burned  per  hour. 

If  we  put  in  (4)  the  probable  values  of  t  for 
the  various  conditions  under  which  the  fan  of  a 


5O  MECHANICAL   DRAFT. 

mechanical  draft  apparatus  must  handle  the  air 
or  gases  of  combustion  we  get 


4C,  for  forced  draft ; 
6C,  for  induced  draft  with  an 
(  5  )  economizer ; 

8C,  for  induced  draft  without 
an  economizer. 


=  A 


Leakage  increases  the  volume  of  air  or  gases 
of  combustion  the  fan  must  handle.  In  the  case 
of  forced  draft  of  the  closed  ash-pit  system  all 
the  air  which  passes  through  the  fan  does  not 
•enter  the  furnace  because  of  the  leakage  outward 
from  the  ash-pit  through  the  walls  of  the  setting 
and  through  the  cracks  of  the  ash-pit  doors. 
And  in  the  case  of  an  induced  draft  system  the 
fan  must  handle  all  of  the  air  which  leaks  into 
the  furnace  or  flues  and  mingles  with  the  prod- 
ucts of  combustion.  The  leakage  is  in  all  cases 
greater  for  a  high  draft  'or  pressure  than  for  a 
low  one.  By  exercising  care  in  the  erection  of  a 
boiler  plant  the  leakage  may  be  made  quite  insig- 
nificant, although  there  are  many  plants  in  which 
the  leakage  probably  amounts  to  fully  15  or  20 
per  cent  of  the  total  volume  of  the  gases  passing 
out  of  the  chimney. 


FUEL   AND   AIR.  51 

A  factor  of  safety  should  of  course  be  used  in 
this  work  as  in  all  engineering  work  and  it  is  well 
to  introduce  it  when  determining  the  value  of  C, 
as  all  the  calculations  are  based  upon  the  number 
of  pounds  of  coal  which  must  be  burned  per  hour. 


CHAPTER  V. 

DRAFT. 

Relation  to  Rate  of  Combustion.  The  draft 
necessary  for  the  combustion  of  a  given  amount 
of  coal  depends  upon  the  velocity  of  the  escaping 
gases  of  combustion,  the  friction  of  the  air 
through  the  fuel  into  the  furnace,  and  the  fric- 
tion of  the  gases  of  combustion  through  the 
various  passages  from  the  furnace  to  the  top  of 
the  chimney.  When  there  is  no  flow  of  air  into 
the  furnace  and  no  flow  of  gases,  out,  the  draft 
necessary  is  zero,  because  there  is  no  friction 
when  there  is  no  velocity.  The  pressure  or  the 
draft  necessary  to  overcome  the  friction  increases 
as  the  square  of  the  velocity  of  the  gases  flowing 
out,  and  hence  as  the  square  of  the  velocity  of 
the  entering  air.  And  since  the  combustion  per 
square  foot  of  grate  surface  is  directly  propor- 
tional to  the  velocity  of  the  entering  air,  the  draft 
necessary  is  directly  proportional  to  the  square 
of  the  number  of  pounds  of  coal  burned  per  hour 
52 


DRAFT.  53 

per  square  foot  of  grate  surface.  Or  to  put  it 
another  way,  the  number  of  pounds  of  coal 
burned  per  square  foot  of  grate  surface  is  directly 
proportional  to  the  square  root  of  the  draft. 

Let  F  be  the  number  of  pounds  of  fuel  burned 
per  square  foot  of  grate  surface  per  hour;  i,  the 
draft  in  inches  of  water  measured  at  the  end  of 
the  uptake  just  where  it  enters  the  chimney; 
and  &,  a  factor  whose  value  depends  upon  the 
kind  of  fuel  and  its  condition  of  fineness  :  then 


(6)  F 

and 

F2 

(7)  i-= 


The  -results  of  many  tests  show  the  rate  of 
combustion  per  square  foot  of  grate  surface  for 
grates  of  different  kinds,  for  coals  of  various 
grades  and  degrees  of  fineness,  and  different 
drafts,  but  they  seem  to  have  been  made  under 
very  different*  conditions  and  therefore  it  is  diffi- 
cult to  arrive  at  the  value  of  k  for  a  particular 
kind  of  coal  under  a  given  set  of  conditions. 
Again  it  will  often  be  found  that  under  appar- 
ently exactly  the  same  conditions,  when  using  the 


54  MECHANICAL   DRAFT. 

same  boiler,  furnace,  and  chimney,  and  the  same 
coal,  a  higher  rate  of  combustion  will  be  obtained 
with  a  low  than  with  a  high  draft.  Such  results 
are  only  explainable  by  the  supposition  that  when 
the  high  draft  was  had,  the  damper  between  the 
chimney  and  the  boiler  was  partly  closed  so  that 
the  full  effect  of  the  draft  was  not  felt  in  the  fur- 
nace. This  means,  of  course,  that  when  going 
over  a  series  of  tests  made  by  an  observer  on  a 
boiler  plant  with  a  particular  kind  of  coal,  care 
must  be  taken  to  select  only  the  highest  rate  of 
combustion  for  a  particular  draft,  and  use  that 
rate  to  obtain  a  proper  value  of  k  in  equations 
(6)  and  (7). 

Hutton.  Hutton  in  his  book,  Steam  Boiler 
Designing,  gives  a  table  showing  the  drafts  nec- 
essary for  the  efficient  combustion  of  different 
fuels,  but  the  table  conveys  very  little  informa- 
tion and  is  of  practically  no  aid  to  an  engineer 
when  designing  a  plant,  because  it  gives  no  inti- 
mation at  all  as  to  the  number  of  pounds  of  each 
fuel  that  may  be  burned  per  square  foot  of  grate 
surface  per  hour  with  the  draft  which  is  stated 
in  the  table  to  be  suitable  to  it.  Thus  he  gives 
0.20  inches  of  water  as  the  draft  required  for 
straw,  and  1.2  to  1.4  as  that  required  for  round 


FIG.    6. — INDUCED  DRAFT  APPARATUS,    NEPTUNE 

CONSUMERS'  ICE  Co.,  BROOKLYN,  N.  Y. 

(American  Blower  Co.) 


DRAFT.  55 

anthracite.  But  whether  it  is  possible  to  burn  10 
pounds  of  straw  and  20  pounds  of  anthracite  per 
hour  per  square  foot  of  grate  surface,  or  vice 
versa,  there  is  no  way  of  telling  from  any  ex- 
planation given  with  the  table.  Hutton  also  gives 
a  table  showing  the  draft  obtained  with  chimneys 
of  different  heights,  and  the  number  of  pounds 
of  coal  that  may  be  burned  per  hour  per  square 
foot  of  grate  surface  with  the  given  drafts.  The 
drafts  are  calculated  on  the  supposition  that  the 
draft  in  inches  of  water  is  equal  to  0.00729  times 
the  height  of  the  chimney  in  feet.  If  k  be  calcu- 
lated from  the  table  it  is  found  that  its  value 
varies  from  about  23  for  chimneys  60  and  80  feet 
high,  assumed  to  give  drafts  of  0.44  and  0.58  of 
an  inch,  to  about  55  for  a  chimney  225  feet  high, 
assumed  to  give  a  draft  of  1.64  inches. 

Thurston  in  his  book,  Manual  of  the  Steam 
Boiler,  gives  certain  formulas  showing  the  rela- 
tion between  the  rate  of  combustion  per  square 
foot  of  grate  surface  and  the  height  of  a  chim- 
ney. Assuming  that  the  draft  in  inches  of  water 
is  ordinarily  something  between  0.005  an^  0.007 
times  the  height  of  the  chimney  in  feet,  the  writer 
finds  that  modifications  of  Thurston's  formulas 
give  for  anthracite  under  best  conditions 


56  MECHANICAL    DRAFT. 

F=  (28  to  24)  VT—  i 
and  for  anthracite  under  ordinary  conditions 
F=  (21  to  18)  VT—  i 

And  for  "  the  best  Welsh  and  Maryland  semi 
anthracite  or  good  bituminous  and  semi-bitumiv.- 
ous  coals," 


F  =  (31  to  27) 

For  "  the  less  valuable  soft  coals," 

F  =  (42  to  36)   A/I"—  i 

Whitham's  Tests.  Whitham  in  a  paper  read 
before  the  American  Society  of  Mechanical  En- 
gineers in  1896,*  gives  a  table  showing  the  rate 
of  combustion  per  square  foot  of  grate  surface 
per  hour  under  different  conditions  when  using 
a  good  quality  of  bituminous  coal  with  a  return 
fire  tube  boiler  and  hand  firing.  From  this  table 
by  Whitham,  Table  IV  has  been  obtained.  The 
values  of  k  given  in  Table  IV  have  been  calcu- 
lated by  the  writer  from  the  two  other  items 
which  have  been  taken  from  Whitham's  table. 


*Trans.   A.   S.    M.    E.,   Vol.   XVII,   1896. 


DRAFT. 


57 


Table  IV  shows  a  gradually  increasing  value 
of  k  with  an  increase  in  the  intensity  of  the  draft, 
which  is  not  what  we  should  expect,  and  is  prob- 

TABLE  IV. 
Whitham's   Tests. 


Rate  of 
Combus- 
tion. F 

Draft. 
i 

k 

5 

0.08 

17.7 

8 

0.16 

20.  0 

10 

0.2O 

22.4 

12 

0.24 

24.5 

14 

0.29 

26.0 

15 

0.31 

26.  Q 

16 

0.33 

27.8 

18 

0.36 

30.0 

20 

0.40 

31-6 

22 

0.44 

33-2 

25 

0.49 

35-7 

28 

0.53 

38-5 

30 

0.57 

39-7 

34 

0.63 

42.8 

36 
40 

0.67 
0.74 

44.0 
46.5 

ably  due  to  less  air  being  admitted  to  the  furnace 
per  pound  of  coal  at  the  higher  drafts  than  at  the 
lower. 


5$  MECHANICAL   DRAFT. 

The  actual  results  of  the  experiments  upon 
which  Whitham  bases  his  table  are  given  with 
the  corresponding  values  of  k,  as  calculated  by 
the  writer,  in  Table  V. 

TABLE  V. 
Whitham's    Tests. 


Rate     of 
combus- 

Draft 

k 

tion.  F 

i 

6.49 

0.16 

16.2 

8.89 

0.21 

19.4 

12.13 

0.33 

21.  1 

16.35 

0.36 

27.2 

19.20 

0.42 

29.6 

20.87 

0.51 

29.2 

26.55 

0.65 

33-0 

30.10 

0.62 

38.2 

34-30 

0.67 

41.8 

Goss*  Tests.  In  1901  Goss  *  in  a  paper  de- 
scribing some  -tests  made  on  the  locomotive  in 
the  laboratory  of  Perdue  University  gives  the 
rates  of  combustion  observed  for  35  tests  for 
which  the  draft  varied  from  1.72  inches  to  7.48 


Trans.     A.  S.  M.  E.,Vol.   XXII,  1901. 


DRAFT.  59 

inches.  The  lowest  draft  gave  a  rate  of  com- 
bustion of  coal  per  hour  per  square  foot  of  grate 
surface  of  49.3  pounds ;  and  the  highest  gave  a 
rate  of  181.6  pounds.  The  coal  used  was  Brazil 
block,  and  the  lowest  rate  of  combustion,  45.9 
pounds,  was  obtained  with  a  draft  of  1.93  inches. 
As  a  result  of  his  experiments  Goss  gives  a  pure- 
ly imperical  formula,  of  not  even  rational  form, 
to  express  the  relation  between  draft  and  rate  of 
combustion,  as  follows : 

i  =  0.037  F 

Wagner's  Tests.  Wagner  discussing  the 
paper  by  Goss  gives  the  results  of  eight  tests 
made  on  a  locomotive  to  determine  the  relation 
between  draft  and  rate  of  combustion.  Four  of 
these  tests  were  shop  tests,  that  is  they  were 
made  in  the  shop ;  and  four  were  road  tests,  made 
with  the  locomotive  under  actual  working  condi- 
tions. In  the  shop  tests  the  draft  varied  from 
2.9  to  3.8  inches  while  the  rate  of  combustion 
varied  from  104.8  to  110.3.  For  the  average  rate 
of  combustion  and  the  average  draft  of  the  shop 
tests  k  is  about  58. 

The  results  of  the  road  tests  together  with  the 


6o 


MECHANICAL    DRAFT. 


value  of  k  as  calculated  by  the  writer  are  given 
in  Table  VI. 

TABLE  VI. 

Wagner's  Tests. 


Rate   of 
combus- 

Draft. 

k 

tion.  F 

i 

23.0 

0.92 

24.0 

52.3 

2.80 

31.3 

56.4 

v       3-23       . 

31.4 

99.6 

5.15 

43-9 

Value  of  k.  After  a  careful  study  of  the  re- 
sults of  a  large  number  of  boiler  trials  and  as 
the  result  of  his  own  experience  and  experiments 
the  writer  is  of  the  opinion  that  it  is  safe  to  give 
to  k  the  following  values  for  boilers  without 
economizers  or  air  heaters,  with  ordinary  sta- 
tionary grates  having  about  50  per  cent  air  space : 


k  = 


34  for  bituminous  block ; 
28  for  bituminous  slack ; 
24  for  anthracite  nut ; 
20  for  anthracite  slack. 


Resistance  of  Grate.     Writers  have  variouslv 


DRAFT.  6 1 

estimated  the  resistance  to  the  flow  of  the  air 
through  the  fuel  on  the  grate  to  be  from  0.4  to- 
0.75  of  the  total  draft  necessary,  for  the  combus- 
tion of  the  coal  at  the  required  rate.  The  resist- 
ance, of  course,  depends  upon  the  velocity  of  the 
entering  air,  and  hence  upon  the  volume  of  air 
admitted  per  pound  of  coal  burned  and  the  rate 
of  combustion.  It  also  depends  upon  the  thick- 
ness of  the  bed  of  fire  and  ashes,  being  less  for 
a  thin  than  for  a  thick  fire.  It  is  greater  with 
a  clinkering  bituminous  than  with  an  anthracite 
coal;  and  it  may  often  be  very  materially  reduced 
by  a  free  use  of  the  slice  bar. 

Whitham,*  in  the  experiments  already  referred 
to,  measured  the  resistance  to  the  flow  of  air 
through  the  fuel  on  the  grate,,  which  was  of  the 
stationary  type,  herring-bone  pattern,  with  air 
openings  equal  to  46  per  cent  of  the  grate  sur- 
face, and  found  that  this  resistance  varied  from 
0.44  to  0.62  of  the  total  draft,  and  as  an  average 
was  about  one-half  the  total  draft.  The  results 
of  Whitham's  experiments  with  the  ratio  of  the 
grate  resistance  to  the  total  draft  are  given  in 
Table  VII. 


*  Trans.   A.   S.   M.   E..  Vol.   XVII,   1896. 


62 


MECHANICAL   DRAFT. 


TABLE  VII. 
Resistance  of  Grate. 


Grate 

, 

Grate 

Total 

resistance 

resistance. 

draft 

divided    by 

total   draft. 

0.7 

0.16 

0.44 

0.13 

O.2I 

0.02 

0.17 

0-33 

0.52 

0.19 

0.36 

0-53 

0.24 

0.42 

0.57 

0.25 

0.51 

0.55 

0.36 

0.65 

0.49 

0.30 

0.62 

0.48 

0.32 

0.67 

0.47 

Average,                             0.52 

It  is  safe  to  assume  that  the  draft  necessary 
to  overcome  the  resistance  to  the  flow  of  the  air 
through  the  coal  of  a  grate  of  the  ordinary  sta- 
tionary type  having  about  50  per  cent  air  space, 
is  0.6  of  the  total  draft  required  for  the  com- 
bustion of  the  coal.  And  hence  in  the  case  of  a 
closed  ash-pit  system  of  forced "  draft  the  fan 
must  be  capable  of  maintaining  in  the  ash-pit  a 
pressure  equal  to  0.6  of  the  total  draft  as  deter- 
mined by  equation  (7)  ;  and  at  least  0.4  of  the 


DRAFT.  63 

total  draft  must  be  produced  by  a  suitable  chim- 
ney. 

Resistance  due  to  Economizer.  It  is  ex- 
tremely difficult  to  determine  the  resistance  to 
.  the  flow  of  the  gases  due  to  the  tubes  of  an 
economizer,  because  of  the  few  tests  that  are  get- 
at-able.  It,  of  course,  depends  upon  the  arrange- 
ment of  the  tubes  more  than  anything  else.  If 
the  tubes  be  arranged  so  that  the  velocity  through 
the  economizer  is  high  then  the  resistance  will 
be  greater  than  if  they  be  arranged  so  that  the 
velocity  is  low.  The  few  tests  to  which  the 
author  has  had  access  indicate  that  the  resistance 
due  to  the  economizer  may  be  even  as  high  as  70 
per  cent  of  the  total  draft,  and  that  ordinarily  it 
is  between  30  and  40  per  cent  of  the  total  draft. 
That  is,  if  the  draft  be  measured  beyond  the 
economizer,  on  the  side  near  the  fan  or  chimney, 
and  be,  say,  0.50  of  an  inch,  we  should  expect  a 
draft  of  about  0.3  to  0.35  of  an  inch  between  the 
economizer  and  the  boiler.  The  draft  between 
the  economizer  and  the  boiler  is  the  draft  which 
is  available  for  overcoming  the  resistance  of  the 
air  through  the  grate,  and  of  the  hot  gases  on 
their  way  from  the  furnace  to  the  economizer. 
And  since  the  resistance  of  the  economizer  is 


64  MECHANICAL  DRAFT. 

about  one-third  the  total  draft  measured  beyond 
the  economizer,  we  may  say  that  the  draft  neces- 
sary with  an  economizer  is  about  50  per  cent 
greater  than  that  necessary  without  an  econo- 
mizer. This  does  not  mean  that  an  economizer 
can  never  be  put  into  a  plant  without  increasing 
the  height  of  the  chimney,  because  often  the 
draft  given  by  the  chimney  is  much  greater  than 
is  necessary. 

The  effect  of  an  economizer  on  the  draft  of  an 
old  plant  is  to  make  it  less  by  an  amount  depend- 
ing entirely  upon  the  decrease  of  the  temperature 
of  the  gases  in  the  chimney.  If  the  economizer 
reduces  the  temperature  of  the  gases  to  between 
300  and  350  degrees,  the  reduction  of  the  draft 
produced  by  a  chimney  of  a  given  height  will  be 
about  15  per  cent,  if  the  original  temperature  of 
the  gases  without  the  economizer  were  about  400 
degrees,  25  per  cent  if  the  original  temperature 
were  about  500  degrees,  and  35  per  cent  if  the 
original  temperature  were  about  600  degrees. 
This  means  that  if  a  plant  has  a  chimney  which 
without  an  economizer  gives  a  draft  of,  say,  0.6 
of  an  inch  with  a  temperature  of  the  gases  of 
about  500  degrees,  we  should  expect  the  draft 
between  the  chimney  and  the  economizer  to  be 
only  about  0.40  to  0.45  of  an  inch  after  the  econ- 


OF  THE 

[UNIVERSITY 

or 


FIG.  7. — ECONOMIZER,  ATLANTA  CONSOLIDATED  STREET 
RAILWAY  Co.,  ATLANTA,  GA. 
(Green  Fuel  Economizer  Co.) 


DRAFT.  65 

omizer  is  put  in.  And  we  should  expect  that  of 
.this  draft  of  0,4  or  0.45  of  an  inch  about  one- 
third  would  be  used  in  overcoming  the  resistance 
clue  to  the  economizer,  leaving  a  draft  of  about 
0.3  of  an  inch  between  the  economizer  and  the 
boiler.  In  this  particular  case,  then,  the  avail- 
able draft  measured  near  the  boiler  has  been  re- 
duced from  about  0.6  to  about  0.3  of  an  inch 
by  the  introduction  of  an  economizer.  And 
whether  or  not  the  use  of  the  economizer  will- 
reduce  the  capacity  of  the  plant  depends  entirely 
upon  whether  the  draft  available  at  the  boiler  is 
less  than  that  necessary  for  the  combustion  of 
the  required  amount  of  coal.  If  the  available 
draft  be  less  than  that  necessary,  the  rate  of  com- 
bustion per  hour  per  square  foot  of  grate  surface 
will  be  decreased,  and  the  capacity  of  the  plant 
will  be  decreased ;  while  if  the  available  draft  be 
greater  than  that  necessary,  the  capacity  of  the 
plant  will  not  be  decreased1  by  the  use  of  an 
economizer,  and  the  cost  of  running  the  plant 
will  usually  be  made  less  than  before  the  econo- 
mizer was  put  in. 

Hence,  when  designing  a  mechanical  draft  ap- 
paratus, we  first  determine  the  draft  necessary 
without  an  economizer  for  the  rate  of  combustion 
desired,  and  then  increase  this  by  50  per  cent  to 


66 


MECHANICAL   DRAFT. 


get  the  draft  necessary  for  the  same  rate  of  com- 
bustion when  an  economizer  is  used.  That  is,  to 
determine  the  draft  necessary  for  a  given  rate  of 
combustion  with  an  economizer,  multiply  the 
value  of  i  as  given  by  equation  (7)  by  1.5. 


TABLE  VIII. 
Induced   Draft   Necessary  without  an   Economizer. 


Necessary  draft,  i,  in  inches  of  water. 

Rate   of 

^Combus- 

Bituminous  Coal. 

Anthracite    Coal. 

tion.  F 

Block. 

Slack. 

Nut. 

Slack. 

5 

O.O2 

0.03 

0.04 

0.06 

10 

0.09 

0.13 

0.17 

0.25 

15 

O.2O 

0.29 

0-39 

0.56 

20 

0-35 

0.51 

0.70 

I.OO 

25 

0.54 

0.81 

i.  08 

1.56 

30 

0.78 

1.  15 

1.56 

2.25 

35 

1.  06 

1.56 

2.12 

3-  06 

40 

1-39 

2.04 

2.78 

4.00 

45 

1-75 

2.58 

3.52 

5.08 

50 

2.16 

3.20 

4-34 

6.25 

Draft  Required  under  Different  Conditions. 
Table    VIII    gives    the    draft    as    calculated    by 


DRAFT. 


67 


equation  (7)  which  must  be  maintained  in  the 
up-take  of  a  boiler  plant  without  an  economizer 
by  the  fan  of  an  induced  draft  apparatus  for  dif- 
ferent coals  when  burned  on  an  ordinary  station- 
ary grate  having  about  50  per  cent  air  space,  at 
various  rates  of  combustion  in  pounds  per  hour 
per  square  foot  of  grate  surface. 


TABLE  IX. 
Induced  Draft  Necessary  with   an   Economizer. 


Necessary  draft,  i,  in  inches  of  water. 

Rate   of 

Combus- 

Bituminous  Coal. 

Anthracite    Coal. 

tion.  F 

Block. 

Slack. 

Nut. 

Slack. 

5 

0.03 

0.05 

0.07 

0.09 

10 

0.13 

0.19 

0.26 

*    o.?8 

15 

0.29 

0.43 

Q.58 

0.84 

20 

0.52 

0.76 

1.04 

1.50 

25 

0.81 

1.19 

1.62 

2-34 

30 

I.i7 

1.72 

2-34 

3-37 

35 

1.59 

2-34 

3-18 

4-59 

40 

2.08 

3.06 

4.16 

6.00 

45 

2.62 

3-87 

5-27 

7  60 

50 

3-25 

4.78 

6.50 

9.36  • 

Table  IX  gives  the  draft  which  must  be  main- 
tained in  the  up-take  of  a  boiler  with  an  econo- 


68  MECHANICAL    DRAFT. 

mizer  by  the  fan  of  an  induced  draft  apparatus 
for  different  coals  when  burned  on  an  ordinary 
stationary  grate  having  about  50  per  cent  air 
space,  at  various  rates  of  combustion  in  pounds 
per  hour  per  square  foot  of  grate  surface.  This 
table  is  obtained  by  multiplying  the  draft  given 
by  equation  (7)  for  a  given  rate  of  combustion 
by  the  factor  1.5. 

Table  X  gives  the  pressure  which  must  be 
maintained  in  the  ash-pit  of  a  closed  ash-pit 
system  of  mechanical  draft  in  order  to  just  over- 
come the  resistance  to  the  friction  of  the  air 
passing  through  the  fire  on  an  ordinary  station- 
ary grate  with  about  50  per  cent  air  space,  when 
burning  different  coals  at  different  rates  of  com- 
bustion in  pounds  per  hour  per  square  foot  of 
grate  surface.  This  table  is  obtained  by  multi- 
plying* the  draft  given  by  equation  (7)  by  the 
factor  0.6. 

It  must  be  remembered  that  when  using  a 
closed  ash-pit  system  of  forced  draft  without 
economizers  it  is  necessary  to  provide  a  chimney 
of  sufficient  height  to  give  a  draft  equal  to  at 
least  0.4  of  the  total  draft,  or  two-thirds  of  the 
draft  given  by  Table  X  for  a  given  rate  of  com- 
bustion for  a  particular  kind  and  quality  of  coal. 


DRAFT. 


TABLE  X. 

Forced  Draft,   Pressure  Necessary,   Closed  Ash-pit 
System. 


Necessary  pressure  i,  in  inches  of  water. 

Rate  of 

Combus- 

Bituminous Coal. 

Anthracite    Coal. 

tion,  r 

Block. 

Slack. 

Nut. 

Slack. 

5 

O.O2 

0.02 

TO 

0.05 

0.08 

0.03 

0.04 

15 

O.I2 

0.17 

O.IO 

0.15 

20 

0.21 

0.31 

0.23 

0-34 

25      . 

0.32 

0.48 

0.42 

0.60 

30 

0.47 

0.68 

0.65 

0.94 

35 

0.64 

0.94 

0.94 

1-35 

40 

0.83 

1.22 

1.28 

1.84 

45' 

1.05 

1-55 

1.67 

50 

1-30 

1.  91 

CHAPTER  VI. 

ECONOMIZERS. 

Effect  of  Adding.  The  addition  of  an  econo- 
mizer to  a  boiler  plant  is  equivalent  to  increasing 
the  heating  surface  of  the  plant  and  always  re- 
sults in  a  decrease  in  the  temperature  of  trie 
escaping  gases  and  an  increase  in  the  tempera- 
ture of  the  feed  water,  and  it  is  the  heat  given 
to  the  feed  water  to  which  the  economy  in  the 
use  of  the  economizer  is  due.  The  economy  is 
two  fold:  first,  more  heat  is  taken  from  the 
.products  of  combustion;  and,  second,  because  of 
the  higher  temperature  at  which  the  feed  water 
enters  the  boiler,  the  evaporation  in  pounds  of 
water  from  and  at  212  degrees  per  hour  per 
square  foot  of  heating  surface  is  reduced,  thus 
bringing  about  an  increase  in  the  efficiency  of 
the  boiler,  or  the  weight  of  water  evaporated 
from  and  at  212  degrees  per  pound  of  coal.  And 
hence  the  saving  in  coal  due  to  the  use  of  an 
economizer  is  usually  greater  than  the  saving  in 
70 


ECONOMIZERS.  /I 

heat  actually  required  for  the  evaporation  of  the 
water  under  the  two  conditions. 

To  make  this  more  apparent  let  it  be  supposed 
that  an  economizer  is  to  be  put  in  a  plant  which 
is  required  to  evaporate  30,000  pounds  of  water 
per  hour  from  an  initial  temperature  of  120  de- 
grees and  under  a  boiler  pressure  of  100  pounds. 
This  is  equivalent  to  33,200  pounds  of  water  per 
hour  from  and  at  212  degrees.  Let  it  also  be 
supposed  that  the  coal  used  has  a  heating  power 
of  13,000  heat  units,  and  that  the  boilers  are  of 
the  water  tube  type  working  at  the  rate  of  4 
pounds  of  water  from  and  at  212  degrees  per 
hour  per  square  foot  of  heating  surface.  From 
equation  (3)  we  have  that  the  weight  of  water 
evaporated  from  and  at  212  degres  per  pound  of 
coal  is 

e 
w  =  z 

2 

4 

=  Z =  Z 2 

2 

From  Table  II  we  find  that  z  for  a  coal  having 
a  heating  power  of  13,000  heat  units  used  with  a 
water  tube  boiler,  is  11.7.  Hence, 

w  =  11.7  —  2  =  9.7 


72  MECHANICAL    DRAFT. 

Therefore,  the  coal  required  per  hour  without 
the  economizer  is 

33200 


,  I     9'7  W 

If  an  economizer  of  proper  size^  be  put  in  it 
would  probably  be  safe  to  say  that  the  water'; 
would  enter  the  boiler  at  225  degrees  instead  of 
1  20.  That  is,  the  temperature  of  the  feed  water1 
would  be  raised  from  120  to  225  degrees  during' 
its  passage  through  the  economizer.  Then  the' 
boiler  would  evaporate  30,000  pounds  of  water 
per  hour  from  an  initial  temperature  of  225  de- 
grees, under  a  boiler  pressure  of  100  pounds; 
which  would  be  equivalent  to  evaporating  29,800 
pounds  of  water  per  hour  from  and  at  212  de- 
grees. The  rate  of  evaporation  then  in  pounds' 
of  water  from  and  at  212  degrees  per  square  foot 
of  boiler  heating  surface  would  be  not  4,  but 

29800  X  4 

=  3.6 


33200 

From  equation  (3)  and  Table  II  we  find  that 
for  a  water  tube  boiler  and  a  coal  having  a  heat- 
ing power  of  13,000,  the  weight  of  water  evap- 


ECONOMIZERS.  73 

orated  from  and  at  212  degres  per  pound  of  coal 
for  a  rate  of  evaporation  per  square  foot  of 
boiler  heating  surface  of  3.6  pounds,  is 

3-6 
w  =-11.7 — 


=  11.7 —  1.8  =  9.9 

And  the  coal  used  per  hour  with  the  econo- 
mizer would  be 

29800 
C=  =3010 

9-9 

The  per  cent  of  saving  of  coal  by  the  use  of  the 
economizer  would  be 

100  (3420  —  3010) 

. .  =  I2.O 


3420 

The  per  cent  of  saving  in  heat  that  must  be 
supplied  by  the  boiler  is  the  same  as  the  per  cent 
of  saving  of  water  from  and  at  212  degrees 
which  the  boiler  must  evaporate;  and  hence  the 
per  cent  of  saving  in  heat  which  the  boiler  must 
supply  would  be 

100  (33200  —  29800) 

— -=  IO.2 
332OO 


74  MECHANICAL    DRAFT. 

In  this  particular  instance  the  per  cent  of  sav- 
ing of  coal  due  to  the  economizer  is  increased  by 
1.8  simply  because  of  the  reduction  of  the  evap- 
oration per  hour  per  square  foot  of  boiler  heating 
surface.  In  other  words,  in  this  particular  case, 
since  1.8  is  about  0.15  of  12.0,  it  is  seen  that 
about  one-seventh  of  the  total  economy  due  to  the 
use  of  the  economizer  is  due  to  the  less  rate  of 
evaporation  per  square  foot  of  boiler  heating 
surface. 

Instead  of  being  used  to  reduce  the  cost  of 
evaporating  a  given  amount  of  water  per  hour, 
an  economizer  may  be  used  to  bring  about  an 
increase  in  the  steam  producing  capacity  of  a 
plant  without  any  increase  in  the  cost  of  fuel. 
Inasmuch,  however,  as  the  cost  of  an  economizer 
might  be  much  greater  than  the  cost  of  an  addi- 
tional amount  of  boiler  surface  for  a  given  in- 
crease in  steam  producing  capacity,  economizers 
are  seldom  used  except  to  reduce  the  cost  of 
producing  a  given  amount  of  steam  by  utilizing 
such  heat  as  would  otherwise  be  wasted. 

Ordinary  Proportion  and  Cost.  Economizers 
are  usually  designed  and  the  surface  in  them 
proportioned  upon  one  square  foot  of  surface  to 
6  pounds  of  water  actually  evaporated  per  hour 


FIG.  8. — INDUCED  DRAFT  APPARATUS,  STATE  CENTRAL 

HEATING  PLANT,  JEFFERSON  CITY,   Mo. 

(Niagara  Radiator  Co.) 


ECONOMIZERS.  75 

by  the  plant ;  sometimes,  however,  the  ratio  is  as 
low  as  one  square  foot  of  surface  to  7  pounds  of 
water,  and  other  times  as  high  as  one  square  foot 
to  5  pounds  of  water.  The  cost  of  an  economizer 
is  greater  than  the  cost  of  a  boiler  of  the  return 
fire  tube  type  having  the  same  number  of  square 
feet  of  heating  surface,  and  less  than  the  cost  of 
a  boiler  of  the  water  tube  type  with  the  same 
number  of  square  feet  of  heating  surface. 

Increase   of   Temperature   of    Feed   Water. 

It  is  almost  impossible  to  predict  exactly  the  rise 
of  temperature  of  the  feed  water  while  passing 
through  an  economizer,  because  of  the  great 
number  of  factors  involved.  It  depends  upon  the 
initial  temperature  of  the  water,  the  velocity  of 
the  water  through  the  pipes ;  the  number  of 
square  feet  of  economizer  surface  provided  per 
pound  of  water ;  the  degree  of  cleanliness  of  the 
surface  of  the  economizer,  inside  and  outside ; 
the  initial  temperature  of  the  hot  gases ;  the 
weight  or  volume  of  the  gases  coming  in  contact 
with  the  economizer  per  pound  of  water  passing 
through  it ;  and,  finally,  upon  the  velocity  of  the 
gases  passing  over  the  surface.  The  rate  of 
transfer  of  heat  from  the  gases  to  the  water 
seems  to  be  somewhere  between  2  and  3  units 


J  MECHANICAL    DRAFT. 

per  hour  per  degree  difference  of  temperatures 
of  the  gases-  and  the  water. 

Roney,  in  a  paper  on  Mechanical  Draft,*  read 
before  the  American  Society  of  Mechanical  En- 
gineers, gives  a  table  showing  the  initial  and 
final  temperatures  of  both  the  water  and  the  gases 
observed  on  tests  of  economizers  of  nine  plants. 
It  is  not  quite  clear  whether  all  of  the  tests  are 
made  on  different  plants  or  whether  more  than 
one  are  made  on  the  same  plant ;  and,  further, 
there  are  no  data  given  as  to  the  ratio  of  weight 
of  feed  water  to  area  of  surface  of  economizer 
that  would  enable  any  general  conclusions  to  be 
drawn  from  the  tests  which  would  be  of  much 
value  in  designing  a  boiler  plant  with  economizers 
in  order  to  get  a  given  final  temperature  of  feed 
water.  Roney 's  results  are  given  in  Table  XL 

A  study  of  economizers  shows  that  it  is  possi- 
ble to  deduce  a  formula  of  rational  form  which 
will  indicate  the  relation  which  must  exist  be- 
tween the  variables  involved,  but  to  obtain  the 
different  constants  that  are  necessary  to  enable 
such  a  formula  to  be  put  in  useable  condition  is 
very  difficult.  The  writer  gives  in  equation  (8) 
a  formula  for  determining  the  rise  in  tempera- 
ture of  the  feed  water  during  its  passage  through 

*  Trans.  A.  S.  M.  E.,  Vol.  XV,   1894. 


ECONOMIZERS.  77 

the  economizer,  and  in  equation  (9)  a  formula 
to  determine  the  fall  of  temperature  of  the  gases. 
In  these  equations,  ^  is  the  initial  temperature 
of  the  water;  t2,  the  final  temperature  of  the 
water  after  passing  through  the  economizer;  7\, 

TABLE  XI. 
Roney's  Experiments  with  Economizers. 


Temperatures  of 
the  gases. 

Temperatures  of 
the  water. 

Initial. 

Final. 

Initial. 

Final. 

610 

505 
550 

340 

212 
205 

no 
84 
185 

287 
276 
305 

522 

320 

155 

300 

505 
465 
490 

320 
250 
290 

IQO 

180 
165 

300 

295 
280 

495 

190 

155 

320 

595 

299 

130 

3ii 

the  initial  temperature  of  the  hot  gases ;  T2,  the 
final  temperature  of  the  gases ;  B ,  the  number  of 
pounds  of  water  flowing  through  the  economizer 
per  hour  per  square  foot  of  economizer  surface ; 
w,  the  weight  in  pounds  of  the  water  actually 
evaporated  per  pound  of  coal ;  and  a,  the  weight 
in  pounds  of  the  gases  admitted  to  the  econo- 
mizer per  pound  of  coal  burned. 


78  MECHANICAL    DRAFT. 

T.-t, 


(8)        t2  —  V 


B  w 

+ +  0.5 


2.4        0.53 

w(t2  — tt) 

(9)        T.-T^ 

o.25a 

It  may  be  assumed  without  a  great  deal  of 
error,  that  a,  the  weight  of  gases  admitted  to  the 
economizer  per  pound  of  coal  burned,  is  about  20, 
so  that  equations  (8)  and  (9)  become 

T.-t, 
(10) 


0.42  B  +  o.i  w  +  0.5 

(ll)  Tx  —  T2  =  0.2W  (t2  — tj 

In  Table  XII  is  given  the  value  of  the  expres- 
sion 0.42  B  -f-  o.i  w  -\-  0.5  for  different  values  of 
B  and  w. 

Example.  Determine  the  rise  in  temperature 
of  the  water  in  an  economizer  which  has  an 
initial  temperature  of  120  degrees,  when  the  hot 
gases  enter  at  a  temperature  of  500  degrees,  and 
the  economizer  is  proportioned  on  the  basis  of 
one  square  foot  of  surface  to  5  pounds  of  water, 


ECONOMIZERS. 


79 


and  the  coal  used  is  such  that  about  9  pounds 
of  water  are  evaporated  under  actual  conditions 
per  pound  of  coal. 

TABLE  XII. 
Economizer  Factors. 


Values  of  B. 

w 

4 

5 

6 

7 

6 

2.8 

3-2 

3-6 

4.0 

7 

2.9 

3-3 

3.7 

4.1 

8 

3-0 

3-4 

3-8 

4.2 

9 

3-  1 

3-5 

3.9 

4-3 

TO 

3-2 

3-6 

4.0 

4-4 

Here  we  have  B  is  5,  and  zt'  is  9,  and  hence 
\ve  find  from  Table  XII,  that  the  economizer  fac- 
tor or  0.42  B  -f  0.1^  +  0.5  is  equal  to  3.5. 
Therefore, 

T!  —  t±          500  —  120 


3-5  3-5 

=  108 

T±  —  T2  =  o.2w(t2  —  tj 
=  1.8  X  108=  194 

Since  t±  is  120  and  t2  —  tt  is  108,  we  have 
t2=ti  +  108  =  228 


80  MECHANICAL    DRAFT. 

An 
have 


MECHANICAL    DRAFT. 

And  since  T\  —  T2  is  194  and  Tt  is  500,  we 
ave 


T2  =i  T±  —  194 

=  500—194  =  306 


That  is  to  say,  the  water  entering  the  econ- 
omizer at  a  temperature  of  120  degrees  leaves 
at  a  temperature  of  228  degrees,  and  the  gases 
entering  at  a  temperature  of  500  degrees  have 
their  temperature  reduced  by  194  degrees  and 
leave  at  306  degrees. 

An  inspection  of  Table  XII.  shows  that  the 
smaller  B  is,  that  is  the  smaller  is  the  number 
of  pounds  of  water  passing  through  the  econ- 
omizer per  hour  per  square  foot  of  surface,  and 
the  smaller  is  w,  the  number  of  pounds  of  water 
evaporated  under  actual  conditions  per  pound  of 
coal,  the  smaller  is  the  expression 

0.42  B  -f  o.i   w  -f-  0.5 

and,  therefore,  the  greater  is  the  increase  in  the 
temperature  of  the  feed  water  passing  through 
the  economizer.  This  means,  that  the  larger  the 
economizer  for  a  given  plant  and  the  poorer  the 
coal  used,  the  greater  is  the  saving  in  heat  and 
coal  effected  by  the  economizer. 


CHAPTER  VII. 

FANS. 

Type     and     Proportions     of     Fans     Used. 

Among  the  different  types  of  fans  on  the  market 
may  be  mentioned  as  the  principal  ones,  the 
disk  fan,  either  with  straight  or  curved  blades,, 
the  cone  wheel  fan,  and  the  centrifugal  fan. 
And  of  all,  the  centrifugal  type  of  fan  is  the 
only  one  which  is  suitable  for  use  with  a  me- 
chanical draft  apparatus.  As  used  for  mechani- 
cal draft  purposes  the  centrifugal  fans  are  always 
provided  with  a  casing  or  housing  of  steel  plate 
and  therefore  they  are  often  termed  "  steel  plate  " 
fans. 

A  centifugal  fan  *  consists  of  a  fan  wheel  re- 
volving in  a  housing  or  casing  provided  with 
suitable  inlets  for  the  entrance  of  the  air  or 
gases,  and  a  suitable  outlet  through  which  the 


*  For  a  full  description  of  centrifugal  fans 
the  reader  is  referred  to  Centriftigal  Fans,  by  J.  H. 
Kinealy. 

81 


82  MECHANICAL    DRAFT. 

air  or  gases  are  allowed  to  pass  out  and  away. 
The  air  always  enters  at  the  center  of  the  wheel 
and  leaves  at  the  periphery  so  that  the  inlets  are 
in  the  sides  of  the  housing  and  the  outlet  in  the 
scroll,  as  the  part  of  the  housing  surrounding 
the  periphery  of  the  wheel  is  termed.  If  the 
fan  has  but  one  inlet  it  is  termed  an  exhauster ; 
while  if  it  has  two  inlets,  one  on  each  side  of  the 
housing,  it  is  termed  a  blower.  With  the  ex- 
ception of  this  difference  in  the  number  r.f  inlets 
there  is  no  difference  between  an  exhauster  and 
a  blower.  Of  course,  the  details  of  construc- 
tion must  be  arranged  to  fit  the  requirements  of 
the  one  or  two  inlets.  The  wheel  revolves  in 
the  housing  and  is  supported  by  a  shaft  which 
lias  two  bearings.  In  the  case  of  a  blower  there 
is  one  bearing  on  each  side  of  the  housing  in 
front  of  the  inlet,  and  the  air  entering  the  fan 
passes  over  these  bearings.  In  the  case  of  an 
exhauster  both  of  the  bearings  are  on  the  same 
side  of  the  housing,  the  side  opposite  to  the  one 
in  which  the  inlet  is.  The  wheel  of  an  exhauster 
is  said,  therefore,  to  be  overhung,  and  the  air 
or  gas  entering  the  fan  does  not  come  in  con- 
tact with  the  bearings.  This  is  an  important  de- 
tail of  construction  when  the  fan  is  used  with 
a  mechanical  draft  apparatus,  as  since  the  hot 


FANS.  83 

gases  do  not  then  come  in  direct  contact  with ' 
the  bearings  it  is  easier  to  keep  them  cool.  Of 
course,  there  is  no  reason  why  a  fan  with  a 
single  inlet  and  an  overhung  wheel  should  not 
be  used  as  a  blower ;  in  fact  such  fans  are  often 
used  as  blowers,  especially  when  the  gases  han- 
dled are  likely  to  have  some  injurious  effect 
on  the  working  parts  of  the  apparatus. 

The  bearing  next  to  the  wheel  of  a  fan  used 
with  an  induced  draft  apparatus  must  always 
be  water- jacketed  so  that  a  continual  stream  of 
water  may  be  used  to  keep  it  cool.  While  the 
hot  gases  do  not  come  in  direct  contact  with 
the  bearing  they  do  come  in  contact  with  that 
part  of  the  shaft  inside  of  the  housing  and  the 
part  of  the  shaft  in  the  bearing  is  heated  by 
conduction  from  the  hot  part. 

The  pressure  against  which  a  centrifugal  fan 
can  force  air  or  the  suction  or  draft  against  which 
it  can  draw  or  suck  air  can  never  exceed  that 
corresponding  to  the  velocity  in  feet  per  minute 
of  the  tips  of  the  blades  or  floats  of  the  tan 
wheel ;  and  when  a  fan  both  sucks  and  forces 
air  or  gases,  the  sum  of  the  draft  necessary  to 
suck  the  air  and  the  pressure  against  which  it 
is  forced  must  not  exceed  the  pressure  corre- 
sponding to  the  velocity  in  feet  per  minute  of  the 


84  MECHANICAL    DRAFT. 

tips  of  the  floats.  The  pressure  which  a  fan 
will  create  in  its  housing  will  depend  upon  the 
density  of  the  gas  it  is  handling,  the  greater  the 
density  of  the  gas  the  greater  will  be  the  pres- 
sure created  for  a  given  velocity  of  the  tips  of 
the  floats  of  the  fan. 

Fans  of  the  centrifugal  type  are  usually  desig- 
nated according  to  the  number  of  inlets,  the 
shape  of  the  housing,  the  position  of  the  outlet, 
and  the  direction  of  the  flow  of  the  air  leaving  the 
outlet.  If  there  are  two  inlets,  the  fan  is  a 
double  inlet  or  double  admission  fan,  while  if 
there  is  only  one  inlet  and  the  bearings  are  both 
on  the  same  side  of  the  housing,  the  fan  is 
called  a  single  inlet  or  single  admission  fan  with 
overhung  wheel.  If  the  housing  is  completely 
above  the  foundation  the  fan  is  said  to  be  a  full 
housed  fan,  and  if  a  part,  usually  about  one- 
quarter,  of  the  housing  projects  below  the  foun- 
dation so  that  the  wheel  revolves  partly  in  a  pit 
or  depression  the  fan  is  said  to  be  a  three- 
quarter  housed  fan.  If  the  outlet  is  at  the  top 
of  the  housing  the  fan  is  a  top  outlet  fan,  and 
if  at  the  bottom,  the  fan  is  a  bottom  outlet  fan. 
And  finally,  if  the  discharge  is  downward,  the 
fan  is  -a  down  discharge ;  if  upward,  an  up  dis- 
charge ;  and  if  horizontal,  a  horizontal  discharge. 


ECONOMIZER,  CLARK  THREAD  Co.,  NEWARK,  N.  J. 
(Green  Fuel  Economizer  Co.) 


FANS.  85 

Thus  the  fan  in  Fig.  I  has  one  inlet,  all  of 
the  housing  is  above  the  foundation,  the  outlet 
is  at  the  bottom  of  the  housing,  and  the  outlet 
is  placed  so  that  the  discharge  is  in  a  horizontal 
direction,  and  hence  the  fan  is  a  "  full  housed, 
single  admission,  bottom,  horizontal  discharge 
fan."  Fans  are  sometimes  made  with  two  out- 
lets as  shown  in  Fig.  II,  and  are  then  called 
double  discharge  fans.  Such  fans,  however,  are 
not  used  with  mechanical  draft  apparatus,  but 
are  used  very  largely  with  heating  and  ventilat- 
ing apparatus. 

The  different  manufacturers  of  fans  designate 
their  fans  according  to  a  number  which  is  ap- 
proximately equal  to  the  height  in  inches  of  a 
full  housed,  top,  horizontal  discharge  fan.  The 
size  of  the  housing,  however,  has  little  to  do 
with  the  working  of  the  fan,  as  that  depends 
altogether  upon  the  diameter  and  proportions  of 
the  wheel  put  in  the  housing.  For  a  given 
diameter  of  wheel  and  a  given  diameter  of  inlet 
the  width  of  a  fan  wheel  may  be  varied  between 
very  wide  limits  and  not  affect  the  working  of 
the  fan  at  all.  The  width  of  the  wheel  of  what 
is  known  as  a  standard  fan  is  usually  about' 
one-half  the  diameter  of  the  wheel. 

The  diameter  of  the  inlet  of  a  fan  is  almost 


86  MECHANICAL    DRAFT. 

always  in  the  case  of  fans  used  for  mechanical 
draft  work,  equal  to  0.707  of  the  diameter  of 
the  wheel,  although  sometimes  it  is  0.625  the 
diameter  of  the  wheel.  The  fan  with  an  inlet 
equal  to  0.625  of  the  diameter  of  the  wheel  will 
handle  less  air  for  a  given  diameter  of  wheel 
than  a  fan  for  which  the  diameter  of  the  inlet 
is  0.707  of  the  diameter  of  the  wheel.  To  supply 
the  same  volume  of  air  when  working  against 
the  same  pressure  or  draft,  a  fan  whose  ratio 
of  diameter  of  inlet  to  diameter  of  wheel  is 
0.625  wiM  have  a  larger  wheel  than  a  fan  in 
which  the  ratio  is  0.707;  but  the  first  fan  will 
do  the  required  work  slightly  more  efficiently : 
that  is,  it  will  require  somewhat  less  power  to  run 
the  fan,  although  the  'difference  will  be  small. 
The  housing  of  the  fan  with  the  smaller  ratio  of 
diameter  of  inlet  to  diameter  of  wheel  may  not 
occupy  any  more  spare  than  the  housing  of  the 
fan  with  the  larger  ratio. 

Relation  Between  Revolutions  of  Fan  and 
Draft.  The  formulas  used  in  this  work  are  ob- 
tained from  the  general  formulas  for  centrifugal 
fans  given  in  the  work  by  the  author  previously 
referred  to. 

Let  V  be  the  velocity  in. feet  per-minute  of  a 


FANS.  87 

gas  whose  density  is  d;  and  p,  the  pressure  in 
ounces  per  square  inch  corresponding  to  this 
velocity,  then 

For  some  reason  not  known  to  the  author,  pres- 
sures or  drafts  against  which  fans  work  are 
usually  expressed  in  ounces  per  square  inch,  but 
the  author  thinks  that  in  mechanical  draft  work, 
it  is  better  to  express  the  draft  produced  by  a 
fan  in  inches  of  water.  Hence,  since  a  pressure 
or  draft  of  one  ounce  per  square  inch  is  equiva- 
lent to  1.73  inches  of  water,  if  we  let  i  represent, 
as  before,  the' draft  or  pressure  of  a  moving  gas 
we  have 

p= ,and  from  (12) 

1-73 

(13)         V=noo 

The  greatest  pressure  or  draft  which  a  fan 
can  produce  is  that  corresponding  to  the  velocity 
in  feet  per  minute  of  the  tips  of  the  floats  or 
blades  of  the  fan  wheel.  And  if  D  be  the  diame- 
ter of  the  wheel  in  feet,  and  N  the  number  of 
revolutions  made  per  minute  by  the  wheel,  the 
velocity  in  feet  per  minute  of  the  tips  of  the 


88  MECHANICAL   DRAFT. 

floats  will  be  wDN.     Hence,  we  have  from  (13) 

(14)  7rDN=noo    /T 

Vd 
From   which    we   have 

(15)  DN  =  350/1 

A/d 

Equation  (15)  shows,  what  has  been  said  be- 
fore, that  in  order  to  produce  a  given  draft  a 
fan  of  a  given  diameter  must  be  run  at  a  higher 
velocity  when  handling  hot  gases  having  a  low 
density,  than  when  handling  cold  air  having  a 
higher  density. 

The  fan  of  a  forced  draft  apparatus  handles 
air  at  rather  a  low  temperature,  seldom  exceed- 
ing 80  or  85  degrees,  so  that  the  value  of  d  may 
be  taken  as  about  0.073,  tne  density  of  air  at 
a  temperature  of  about  85  degrees. 

The  fan  of  an  induced  draft  apparatus  with 
an  economizer  handles  gases  whose  temperature 
may  be  as  high  as  300  or  350  degrees,  and  the 
value  of  d  may  be  taken  as  about  0.050,  the 
density  of  air  at  a  temperature  of  about  335  de- 
grees. 

The  fan  of  an  induced  draft  apparatus  without 
an  economizer  must  ordinarily  handle  the  gases 
of  combustion  at  a  temperature  between  500  and 


FANS.  89 

600  degrees,  so  that  the  value  of  d  may  be  taken 
as  about  0.039,  the  density  of  air  at  a  temperature 
of  about  560  degrees. 

If  we  put  now  these  values  of  d  in   (15)   we 
have 

1295   \7^~    for  forced  draft ; 
15&5   VT"    f°r  induced  draft  with  econ- 
DN  =  )  omizer  ; 

1775   \/J7     for    induced    draft    without 

economizer. 

For  all  practical  purposes  it  is  exact  enough 
to  say 

1300  \/^~    f°r  forced  draft ; 
1600  \/i7    for      induced      draft 


(16)     DN  = 


with   economizer ; 


1800  \/i7  for  induced  draft 
without  economizer. 
Capacity  of  Fan.  The  capacity  of  a  fan  is  the 
greatest  amount  of  air  in  cubic  feet  per  minute 
it  will  deliver  while  maintaining  a  pressure  in 
the  housing  equal  to  that  corresponding  to  the 
velocity  of  the  tips  of  the  floats  of  the  fan  wheel. 
And  a  fan  is  said  to  be  working  within  capacity 
when  the  amount  of  air  handled  by  it  is  equal 
'to  or  less  than  its  capacity.  As  long  as  a  fan 
is  working  within  its  capacity,  the  pressure  or 


9O  MSCHANICAL   DRAFT. 

draft  produced  by  it  is  equal  to  that  correspond- 
ing to  the  velocity  of  the  tips  of  the  floats  of  the 
wheel.  When  a  fan  is  working  within  or  at  it? 
capacity,  the  theoretical  outlet,  if  the  fan  be 
blowing,  or  the  theoretical  inlet,  if  it  be  suck- 
ing, is  equal  to  what  is  called  the  "  blast  area  '' 
of  the  fan.  By  making  the  theoretical  outlet  in 
the  case  of  a  blower  or  the  theoretical  inlet  in 
the  case  of  an  exhauster,  greater  than  the  "  blast 
area  "  the  fan  can  be  made  to  deliver  more  air 
than  when  working  at  its  capacity,  but  then  the 
fan  will  be  working  at  a  less  efficiency.  On  the 
score  of  economy,  a  matter  of  much  importance 
in  mechanical  draft  apparatus,  it  is  not  advisable 
to  work  a  fan  beyond  its  capacity. 

Let  A  be  the  capacity  of  the  fan  ;  r  the  ratio 
obtained  by  dividing  the  diameter  of  the  inlet  by 
the  diameter  of  the  fan  wheel ;  D  the  diameter 
of  the  fan  wheel  in  feet ;  and  N  the  number  of 
revolutions  at  which  the  fan  is  run.  Then 

(17)         A=i.38r3D3N 

A  double  admission  fan  would  have  a  capacity 
equal  to  twice  that  of  a  single  admission  fan 
with  an  overhung  wheel,  were  it  not  that  usually 
the  inlets  are  so  obstructed  by  the  shaft  and  its 
bearings,  and  the  driving  pulley  on  one  side,  that 


FANS.  91 

the  sum  of  free  and  unobstructed  areas  of  the 
two  inlets  of  a  double  admission  fan  is  seldom 
much  if  any  greater  than  the  area  of  the  single 
inlet  of  a  single  admission  fan  of  the  same  size. 

As  has  been  said  before,  the  usual  value  of  r 
for  fans  used  for  mechanical  draft  is  0.707,  and  if 
this  value  be  put  in  (17)  we  have 

(18)  A  =  o.49D3N 

If  in  (18)  w€  put  for  DN  its  value  as  given  in 
(16)  we  have: 

For  forced  draft, 

(19)  A  =  64oD2  yT 

For  induced  draft  iwth  an  economizer, 

(20)  A  =  78oD2  VF 

For  induced  draft  without  an  economizer, 

(21)  A  " 


CHAPTER  VIII. 

PROPORTIONING    THE    PARTS. 

Diameter  of  Fan  Wheel  Required.  From 
what  has  been  said  before  it  is  known  that  the 
air  to  be  handled  per  minute  for  the  combustion 
of  C  pounds  of  coal  per  hour  with  a  forced  draft 
is  4C.  So  putting  in  (19)  this  value  of  A  and 
solving  for  C  we  have  for  the  relation  between  the 
pounds  of  coal  burned  per  hour  and  the  diameter 
of  the  fan  wheel  for  forced  draft, 

(22)  0=1600^ 

Since  the  gases  to  be  handled  per  minute  for 
the  combustion  of  C  pounds  of  coal  with  an  in- 
duced draft  apparatus  with  an  economizer  is  6C , 
by  substituting  6C  for  A  in  (20)  and  solving 
for  C,  we  have  for  the  relation  between  the 
pounds  of  coal  burned  per  hour  and  the  diameter 
of  the  fan  wheel  of  an  induced  draft  apparatus 
iwth  an  economizer, 

(23)  C 


PROPORTIONING    THE    PARTS.  93 

And  since  the  gases  to  be  handled  per  minute 
for  the  combustion  of  C  pounds  of  coal  per  hour 
with  an  induced  draft  apparatus  without  an 
economizer  is  8C,  by  substituting  8C  for  A  in 
(21)  and  solving  for  C,  we  have  for  the  relation 
between  the  pounds  of  coal  burned  per  hour  and 
the  diameter  of  the  fan  wheel  of  an  induced 
draft  apparatus  without  an  economizer, 

(24)         C=noD2\/T 

Table  XIII,  calculated  from  (22),  shows  the 
number  of -pounds  of  coal  that  can  be  burned 
per  hour  with  a  forced  draft  apparatus  when 
the  fan  is  run  at  a  speed  corresponding  to  differ- 
ent pressures  in  the  ash-pit. 

Table  XIV,  calculated  from  (23),  shows  the 
number  of  pounds  of  coal  that  can  be  burned 
per  hour  with  an  induced  draft*  apparatus  with 
an  economizer  when  the  fan  is  run  at  a  speef] 
corresponding  to  different  drafts. 

Table  XV",  calculated  from  (24),  shows  the 
number  of  pounds  of  coal  which  can  be  burned 
per  hour  with  an  induced  draft  apparatus  without 
an  economizer  when  the  fan  is  run  at  a  speed 
corresponding  to  different  drafts. 


94 


MECHANICAL   DRAFT. 


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PROPORTIONING    THE    PARTS.                          95 

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PROPORTIONING    THE    PARTS.  97 

The  use  of  the  tables  may  be  illustrated  by  an 
example. 

Example.  Determine  the  diameter  of  a  wheel 
of  a  fan  for  an  induced  draft  apparatus  with 
an  economizer  to  burn  4500  pounds  of  coal  per 
hour  under  a  draft  of  about  0.8  of  an  inch. 

Turning  to  Table  XIV  we  see  that  a  6^-foot 
wheel  is  more  than  large  enough  for  4500  pounds 
of  coal  an  hour  under  a  draft  of  0.8  of  an  inch,, 
and  that  a  5-foot  wheel  is  large  enough  under 
a  draft  of  2  inches.  The  5-foot  wheel  would  cost 
more  to  run,  because  it  would  require  more 
power  than  the  6j^-foot  wheel. 

Speed    at    which    the    Fan    must    be    Run. 

When  the  pressure  or  draft  and  the  diameter  of 
the  wheel  have  been  determined,  it  is  then  neces- 
sary to  determine  the  number  of  revolutions  at 
which  the  wheel  must  be  run  in  order  to  give 
the  required  draft. 

For  a  forced  draft  we  get  from  (16) 

1300  yT 
D 

This  equation  has  been  used  to  calculate  Table 
XVI,  which  gives  the  number  of  revolutions 
per  minute  that  wheels  of  various  diameters  used 


98  MECHANICAL    DRAFT. 

for   forced   draft,   must   make   in   order   to   give 
different  pressures  in  inches  of  water. 

For  an  induced  draft  apparatus  ivith  an  econo- 
mizer we  have  from  (16) 


i6oo 

JN  = 


D 

This  equation  has  been  used  to  calculate  Table 
XVII,  which  gives  the  number  of  revolutions 
that  must  be  made  per  minute  by  the  wheels  of 
induced  draft  fans  with  economizers  in  order  to 
give  different  drafts  in  inches  of  water. 

We  have  from  ( 16)  for  an  induced  draft  fan 
zvithout  an  economizer 

N=  '800  vr 


D 

This  equation  has  been  used  to  calculate  Table 
XVIII,  which  gives  the  number  of  revolutions 
that  must  be  made  per  minute  by  the  wheels  of 
induced  draft  fans  without  economizers  in  order 
to  give  different  drafts  in  inches  of  water. 

Power  Required  to  Run  the  Fan.  From 
the  author's  work  on  fans  previously  referred  to 


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102  MECHANICAL    DRAFT. 


we  get  that  the  horse  power,  H,  required  to  drive 
a  fan  when  working  at  its  capacity  is 

Ap(i  +  r2) 
(25)         H  = 


3300 

In  this  equation  A  is  the  cubic  feet  of  air  or 
gas  handled  per  minute  ;  p,  the  pressure  in  ounces 
per  square  inch  corresponding  to  the  velocity  of 
the  tips  of  the  floats  of  the  wheel  ;  and  r,  as  be- 
fore, is  the  ratio  of  the  diameter  of  the  inlet 
divided  by  the  diameter  of  the  wheel.  If  we 

put  for  p  its  value  —  —  and  for  r  the  value  gen- 
erally found  in  fans,  0.707,  we  get 

Ai 
(26) 


38oo 

For  forced  draft  we  know  that  A  is  equal  to 
4C,  where  C  is  the  weight  of  coal  burned  per 
hour.  Hence  if  we  put  in  (26)  for  A  its  value, 
we  get  for  forced  draft, 

Ci 

(27)         H=- 


950 
For  induced  draft  with,  an  economizer  we  know 


PROPORTIONING    THE    PARTS.  IO3 

that  A  is  equal  to  6C  and  hence  we  have  from 
(26)  for  induced  draft  with  an  economizer, 

Ci 

(28) 


635 

For  induced  draft  without  an  economizer  we 
know  that  A  is  equal  to  8C,  and  hence  we  have 
from   (26)    for  induced  draft  zvithotit  an  econ- 
omizer, ~  . 
C  i 

(29)         I 


475 

If  we  put  for  C  in  (27)  its  value  as  given  by 
(22),  we  have  for  forced  draft  fans, 


(30)         H  = 

5-9 

Table  XIX  has  been  calculated  from  (30).  It 
gives  the  horse  power  required  to  drive  forced 
draft  fans  when  working  at  their  capacity  under 
different  pressures  in  inches  of  water. 

If  we  put  for  C  in  (28)  its  value  as  given  by 
(23),  we  have  for  induced  draft  fans  with  econ- 
omizers,  2  F 


4-8 


104 


MECHANICAL   DRAFT. 


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PROPORTIONING   THE    PARTS.  IO/ 

Table  XX  has  been  calculated  from  (31).  It 
gives  the.  horse  power  required  to  drive  induced 
draft  fans  with  economizers  when  working  at 
their  capacity  under  different  drafts  in  inches  of 
water. 

If  we  put  for  C  in  (29)  its  value  as  given  by 
(24),  we  have  for  induced  draft  fans  without 
economizers, 


4-3 


Table  XXI  has  been  calculated  from  (32).  It 
gives  the  horse  power  required  to  drive  induced 
draft  fans  without  economizers  when  working  at 
their  capacity  under  different  drafts  in  inches  of 
water. 

Size  of  Engine  Required.  The  indicated 
horse  power  of  the  engine  is,  as  has  been  said 
before,  equal  to  about  1.5  times  the  horse  power 
required  to  drive  the  fan,  or  1.5  times  the  value 
of  H  as  given  by  (30)  for  forced  draft,  or  by 
(31)  for  induced  draft  with  an  economizer,  or 
by  (32)  for  induced  draft  without  an  economizer. 


108  MECHANICAL    DRAFT. 

From  (16),  (30),  (31),  and  (32),  we  get 
i.SD'Ni 


<33) 


-,  for  forced  draft. 


5-9  X  1300 

i.5D3Ni 

— ,   for  induced  draft 
4.8  X  1600         with    an    econ- 
omizer. 
i.5D3Ni 

,   for  induced  draft 

4.3X1800         without     an 
economizer. 


D3Ni 


-    for  all. 


From  the  well  known  formulas  in  regard  to 
steam  engines  it  is  known  that  if  P  is  the  mean 
effective  pressure  of  the  steam  in  the  cylinder ; 
/,  the  stroke,  in  inches,  of  the  engine ;  d,  the 
diameter,  in  inches,  of  the  cylinder ;  and  N,  the 
number  of  revolutions  made  per  minute  by  the 
engine ;  the  indicated  horse  power  of  the  engine 

is  .     In  the  case  of  an  engine  direct  con- 

252000 

nected  to  a  fan  the  number  of  revolutions,  N, 
made  by  the  engine  is  the  same  as  the  number 
of  revolutions  made  by  the  fan. 


PROPORTIONING    THE    PARTS.  IO9 

But  as  has  been  said,  i-5//  is  equal  to  the  indi- 
cated horse  power  developed  by  the  engine,  and 
hence  for  direct  connected  engines. 

PI  d2N  D3Ni 


252000          5150 

From  this  we  get 

49D3i 
(34)         ld>  = 


The  direct  connected  engines  ordinarily  used 
to  drive  the  fans  of  a  mechanical  draft  apparatus 
cut  off  at  3/4  stroke,  so  that  the  mean  effective 
pressure,  P,  when  the  boiler  pressure  is  about 
loo  pounds  by  the  gage,  may  safely  be  taken  be- 
tween 80  and  90.  If  we  say  P  is  equal  to 
80,  (34)  becomes, 

(35)        ld2  =  o.6iD3i 

in  these  engines  the  stroke  varies  from  I  to  1.5 
times.the  diameter,  but  is  seldom  if  ever  greater 
than  1.5  times  the  diameter.  If  in  (35)  we 
substitute  d  for  /  and  solve  for  d  we  get  the  ex- 
pression for  the  diameter  of  the  engine  required 


no 


MECHANICAL    DRAFT. 


for   the    fan    when    the    stroke   is    equal   to    the 
diameter, 

(36)       d  =  o.85  DVi" 

Table  XXII  gives  the  value    of    0.85  ^/i    for 
different  volues  of  i. 

TABLE  XXII. 
Values  of  0.85^?" 


$ 

o.Ss^T 

0.5 

1.0' 

0.67 
0.86 

1.5 

2.0 

0.96 
i.  06 

2.5 

1.14 

3-0 

I.  21 

This  table  shows  plainly  that  the  common  rule- 
of-thumb  for  determining  the  diameter  of  the 
direct  connected  engine  for  a  fan  to  give  a 
draft  of  from  i  to  1.5  inches  of  water  is  correct. 
The  rule  is :  Make  the  diameter  of  the  engine 
in  inches  about  the  same  as,  but  not  less  than 
three-quarters,  the  diameter  of  the  fan  wheel  in 
feet,  and  make  the  stroke  greater  than  the  diam- 
eter. 


PROPORTIONING    THE    PARTS.  Ill 

Instead  of  using  Table  XXII  to  determine  the 
size  of  engine  required,  equation  (35)  may  be 
used  in  connection  with  Table  XXVII  in  the 
Appendix. 

Steam  Used  by  Fan  Engine.  The  fan  en- 
gines being  rather  inefficient  and  cutting  off  be- 
tween one-half  and  three-quarters  stroke,  use  a 
great  deal  of  steam  per  hour  per  indicated  horse- 
power, so  that  this  steam  used  by  them  may  be 
a  considerable  per  cent  of  the  whole  capacity  of 
the  plant  unless  care  be  taken  in  designing  the 
apparatus  to  keep  the  consumption  within  a  cer- 
tain limit.  It  is  probable  that  the  steam  used 
per  indicated  horse  power  by  the  fan  engines 
will  not  be  far  from  40  to  50  pounds  per  hour. 
And  as  the  indicated  horse  power  is  probably 
not  far  from  1.5  the  power  required  to  run  the 
fans  as  given  by  Table  XIX  for  forced  draft ; 
Table  XX  for  induced  draft  with  an  economizer ; 
and  Table  XXI  for  induced  draft  without  an 
economizer,  we  may  say  that  the  weight  of  steam 
required  per  hour  to  run  a  fan  is  equal  to  the 
power  as  given  by  the  proper  table  multiplied 
by  70. 

Choosing  the  Fan.     When  designing  a  me- 


112  MECHANICAL    DRAFT. 

chanical  draft  plant  it  is  important  to  know, 

(a)  The  kind  of  mechanical  draft,  L  e.,  forced, 
induced    with    economizer,    or    induced    without 
economizer. 

(b)  The  kind  of  coal  to  be  used,  and  the  rat? 
of  combustion  per  hour  per  square  foot  of  grate 
surface. 

(c)  The  maximum  weight  of  coal  to  be  burned 
per  hour,  and  the  probable  actual  evaporation  of 
water  per  pound  of  coal. 

(d)  The  maximum  per  cent  of  the  steam  gen- 
erated that  may  be  used  per  hour  to  run  the  fan 
engine. 

Knowing  these  conditions  we  determine  the 
minimum  allowable  draft,  which  is  the  draft  nec- 
essary for  the  combustion  of  the  given  kind  of 
coal  at  the  required  rate.  Then  by  the  use  of 
the  various  tables  we  determine  the  diameter  of 
the  fan  wheel  which  for  the  kind  of  draft  has  a 
capacity  equal  to  the  maximum  weight  of  coal 
to  be  burned  per  hour,  when  using  not  more 
steam  than  is  allowed  by  the  conditions  of  the 
problem. 

The  course  to  be  followed  when  choosing  a  fan 
for  a  given  set  of  conditions  can  be  best  illus- 
trated by  an  example. 

Example.    Determine  the  proper  size  of  fan  t< 


PROPORTIONING    THE    PARTS.  113 

be  vised  for  an  induced  draft  plant  without  an 
economizer  to  burn  6,000  pounds  of  coal  per  hour 
at  a  maximum  rate  of  24  pounds  of  coal  per  hour 
per  square  foot  of  grate  surface.  The  coal  is  of 
a  low  grade  bituminous,  approaching  the  condi- 
tion of  slack ;  and  the  steam  used  to  run  the  fan 
must  not  exceed  2^/2  per  cent  of  the  steam  made 
by  the  plant. 

From  Table  VIII  we  see  that  a  draft  of  o.8r 
of  an  inch  is  necessary  for  the  combusion  of  25 
pounds  of  bituminous  slack  per  hour  per  square 
foot  of  grate  surface,  and  hence  we  may  assume 
that  the  draft  under  which  the  fan  must  work 
must  not  be  less  than  0.8  of  an  inch. 

It  is  probable  that  the  total  water  evaporated 
per  hour  under  actual  conditions  will  be  about 
35,000  pounds.  And  2^/2  per  cent  of  this  is  825 
pounds,  which  is  the  weight  of  steam  the  engine 
may  use  to  drive  the  fan.  Assuming  70  pounds 
of  water  per  hour  per  horse  power,  we  have  that 
the  horse  power  to  drive  the  fan  must  not  exceed, 

825 

=11.8 

70 

Turning  now  to  Table  XV  we  see  that  we 
may  use  an  8- foot  wheel  at  a  draft  of  0.8  of 
an  inch  or  a  7-foot  wheel  at  a  draft  of  1.25 


114  MECHANICAL   DRAFT. 

inches.  From  Table  XXI  we  see  that  a  10.6 
horse  power  will  be  required  to  run  the 
8-foot  wheel  at  a  draft  of  0.8  of  an  inch,  and 
15.8  horse  power  will  be  required  to  run  the  7- 
foot  wheel  at  a  draft  of  1.25  inches. 

However,  let  us  suppose  that  we  choose  the 
8-foot  fan.  We  now  turn  to  Table  XVIII  and 
find  that  this  fan  must  be  run  at  a  speed  of  200 
revolutions  per  minute  to  give  a  draft  of  0.8 
of  an  inch. 

Turning  now  to  Table  XXII  we  see  that  for 
the  8-foot  fan,  we  may  use  an  engine  whose 
cylinder  diameter  in  inches  is  between  0.86  and 
0.67  of  the  diameter  of  the  wheel  in  feet.  That 
is,  a  7  by  7  or  a  6  by  9  engine  is  large  enough. 

The  process  then  to  be  followed  to  choose  a 
proper  fan  is  as  follows : 

FOR  FORCED  DRAFT. 

1  —  From    Table    X    determine    the    pressure 
which  must  be  maintained  in  the  ash-pit  for  the 
combustion  of  the  given  kind  of  coal  at  the  re- 
quired maximum  rate. 

2  —  From  Table  XIII  choose  a  fan  which  is 
large  enough  for  the  maximum  weight  of  coal 
to  be  burned  when   working  at  a  pressure  not 
less  than  that  required  by  Table  X. 


PROPORTIONING   THE    PARTS.  '115 

3  —  From  Table  XVI  determine  the  number  of 
revolutions  the  fan  must  make  per  minute. 

4  —  From  Table  XIX  determine  the  power  re- 
quired to  run  the  fan. 

5  _  Find  by  the  aid  of  Table  XXII   the  di- 
ameter of  the  cylinder  of  the  engine  required. 

6  —  Multiply  the  horse  power  by  70  to  get  the 
weight  of  steam  required  per  hour  to  run  the  fan. 

FOR  INDUCED  DRAFT  WITH  ECONOMIZER. 

1  —  From   Table   IX   find  the   draft   required 
for  the  given  kind  of  coal  and  rate  of  combustion 

2  —  From    Table    XIV    choose    a    fan    large 
enough    for   the   maximum    rate   of   coal   to  be 
burned  per  hour  when  working  at  a  draft  not  less 
than  that  required  by  Table  IX. 

3  —  From  Table  XVII  determine  the  number 
of  revolutions  the  fan  must  make  per  minute. 

4  —  From    Table    XX    determine    the    horse 
power  required  to  run  the  fan. 

5  — Find  by  the  aid  of  Table  XXII  the  di- 
ameter of  the  cylinder  of  the  engine  required. 

6 —  Multiply  the  horse  power  by  70  to  get  the 
weight  of  steam  required  per  hour  to  run  the 
engine. 


Il6  MECHANICAL  DRAFT. 

FOR  INDUCED  DRAFT  WITHOUT  ECONOMIZER. 

1  —  From  Table  VIII  find  the  draft  required 
for  the  given  kind  of  coal  and  rate  of  combus- 
tion. 

2  —  From  Table  XV  choose  a  fan  large  enough 
for  the  maximum  weight  of  coal  to  be  burned 
per  hour  when  working  at  a  draft  not  less  than 
that  required  by  Table  VIII. 

3  —  From  Table  XVIII  determine  the  number 
of  revolutions  the  fan  must  make  per  minute. 

4  —  From    Table    XXI    determine    the    horse 
power  required  to  run  the  fan. 

5  — Find  by  the  aid  of  Table  XXII  the  di- 
ameter of  the  cylinder  of  the  engine  required. 

6  —  Multiply  the  horse  power  by  70  to  get  the 
weight  of  steam  required  per  hour  to  run  the 
engine. 

It  may  often  happen  that  the  maximum  weight 
of  coal  to  be  burned  per  hour  by  a  plant  will  be 
that  which  will  be  required  only  a  few  days  dur- 
ing the  year.  It  may  well  occur  in  the  case  of 
a  plant  that  supplies  steam  for  power  and  heat- 
ing purposes,  that  an  unusual  amount  of  steam 
may  be  required  for  possibly  a  week  or  two  dur- 
ing the  severe  weather  of  the  winter  and  the  rest 
of  the  year  the  amount  of  coal  that  is  required  to 


PROPORTIONING    THE    PARTS.  117 

be  burned  may  not  be  more  than  two-thirds  of 
three-quarters  of  the  maximum  amount.  In  a 
case  of  this  kind  it  -  will  usually  be  economy  to 
put  in  a  fan  sufficiently  large  to  take  care  of  the 
normal  amount  and  when  the  excessive  demand 
comes  upon  the  plant,  speed  the  fan  up  or  run 
both  fans.  It  is,  of  course,  to  be  understood  that 
in  all  cases  where  an  induced  draft  system  is  in- 
stalled, the  fans  are  to  be  installed  in  duplicate 
and  one  fan  should  be  sufficient  to  take  care  of 
the  plant  with  probably  20  to  25  per  cent  more 
than  the  normal  condition  of  combustion ;  and 
whenever  the  requirements  of  the  plant  demand 
more  than  one  fan  can  do,  both  fans  may  be  run. 

Location  of  the  Fans.  No  general  rule  can 
be  given  as  to  where  the  fans  of  a  mechanical 
draft  apparatus  should  be  located,  as  this  will  de- 
pend upon  the  arrangement  of  the  boilers  in  the 
boiler  house.  The  fans,  however,  may  be  put  on 
the  floor  or  near  the  roof,  but  in  every  case  they, 
should  be  convenient  of  access  so  that  the  en- 
gines can  be  attended  to. 

Breeching  and  Uptake.  The  breeching  and 
uptake  connections  between  the  boilers  and  the 
chimney  of  a  closed  ash-pit  system  of  mechanical 


Il8  MECHANICAL    DRAFT. 

draft  should  be  proportioned  just  as  for  an  ordi- 
nary system  of  chimney  draft. 

In  the  case  of  a  system  of  induced  draft  it  is 
'usual  to  make  the  breeching  and  uptake  connec- 
tions somewhat  smaller  than  for  a  chimney  draft 
of  the  same  intensity,  probably  because  if  a 
higher  draft  should  be  needed  it  is  so  very  easy 
to  get  it  by  speeding  up  the  fans,  in  the  case  of 
mechanical  draft,  while  in  the  case  of  a  chimney 
draft  the  only  way  to  increase  the  intensity  of 
the  draft  for  a  given  set  of  conditions  as  to  tem- 
perature of  outside  air  and  gases  inside,  is  to 
make  the  chimney  higher. 

In  no  case  should  the  area  of  the  breeching 
leading  to  the  fan  be  of  a  less  area  than  the  inlet 
of  the  fan,  and  in  most  cases  it  is  well  to  make  it 
larger.  The  area  of  the  breeching  should  be 
made  greater  in  proportion  to  the  area  of  the 
inlet  for  small  than  for  large  fans.  It  is  ex- 
tremely difficult  to  get  data  as  to  the  proper  sizes 
of  breeching  to  be  used  with  different  sizes  of 
fans,  and  in  fact  the  size  of  the  breeching  should 
be  made  to  depend  not  upon  the  size  of  fan 
but  rather  upon  the  number  of  pounds  of  coal  to 
be  burned  per  hour.  If  the  length  of  the  breech- 
ing, the  number  of  turns  and  bends,  and  the  other 
resistances  to  the  flow  of  gases  are  known  it 


PROPORTIONING    THE    PARTS.  119 

would  be  a  very  simple  matter  to  design  a  breech- 
ing so  that  the  resistance  due  to  friction  may  be 
a  certain  predetermined  amount.  Unfortunately 
it  is  next  to  impossible  to  proportion  breechings 
in  this  way,  and  hence  other  methods  must  be  re- 
sorted to. 

After  as  careful  a  study  of  the  subject  as  the 
data  available  would  allow,  the  author  has  cal- 
culated Table  XXIII  to  be  used  when  propor- 
tioning the  breeching  and  uptake  connections. 
The  table  is  based  upon  the  weight  of  coal  to  be 
burned  per  hour  and  the  draft  to  be  given  by 
the  fan,  and  is  intended  primarily  for  induced 
mechanical  draft  systems  having  square  or  nearly 
square  breeching  and  uptake  connections.  If  the 
breeching  or  uptake  is  to  be  round  it  should  have 
a  diameter  one-tenth  greater  than  the  side  of  the 
square  given  in  the  table,  since  a  circle  whose 
diameter  is  10  per  cent  greater  than  the  side  of 
a  given  square  offers  the  same  resistance  to  the 
flow  through  it  of  a  given  volume  of  gases  per 
hour  as  the  square,  although  the  area  of  the  cross- 
section  of  the  circle  is  about  5  per  cent  less  than 
that  of  the  square. 

The  use  of  the  table  can  best  be  shown  by  an 
example. 

Example.     Determine    the    size    of    breeching 


I2O  MECHANICAL   DRAFT. 

and  uptake  connections  for  an  induced  system  of 
mechanical  draft  for  four  boilers  each  having 
a  furnace  capable  of  burning  1,500  pounds  of  coal 
per  hour  under  a  draft  of  1.25  inches  of  water. 

Here  the  draft  is  1.25  inches,  and  the  total  coal 
to  be  burned  per  hour  is  6,000  pounds.  Hence 
we  look  in  Table  XXIII  down  the  column  show- 
ing a  draft  of  1.25  inches  until  we  find  6,000,  and 
then  find  the  side  of-  the  square  in  the  first  column 
opposite  6,000.  We  find  that  the  breeching  be- 
tween the  four  boilers  and  the  fan'  should  be  a 
square  whose  side  is  62  inches  and  whose  area  is 
3,844  square  inches.  If  the  breeching  be  round 
the  diameter  should  be  about  68  inches. 

Three  of  the  boilers  will  have  burned  under 
them  a  total  of  4,500  pounds  of  coal  per  hour. 
Again  looking  down  the  column  showing  a  draft 
of  1.25  inches  we  find  that  a  square  breeching 
whose  side  is  54  inches  and  whose  area  is  2,916 
square  inches  is  large  enough  for  4,400  pounds 
of  coal  per  hour,  and  hence  we  may  use  this  as 
the  size  of  the  breeching  between  the  third  and 
fourth  boilers. 

The  breeching  between  the  second  and  third 
boilers  must  be  large  enough  to  take  care  of  3,000 
pounds  of  coal  per  hour  under  a  draft  of  1.25 
inches  of  water.  The  table  shows  that  we  may 


PROPORTIONING    THE    PARTS.  I2t 

use  here  a  square  whose  side  is  46  inches  and 
whose  area  is  2,116  square  inches,  as  it  will  be 
large  enough  to  take  care  of  3,100  pounds  of  coal 
per  hour. 

The  breeching  between  the  first  boiler  and  the 
second  must  be  large  enough  to  take  care  of  1,500 
pounds  of  coal  per  hour  under  a  draft  of  1.25 
inches  of  water  and  the  table  shows  that  if  it 
be  square  its  side  should  be  34  inches  and  its 
area  of  cross-section  should  be  1,156  square 
inches. 

Since  the  uptake  connection  from  each  boiler 
to  the  breeching  is  to  be  large  enough  to  take 
care  of  1,500  pounds  of  coal  per  hour  under  a 
draft  of  1.25  inches  of  water,  we  should  give  each 
an  area  of  1,156  square  inches,  and  make  it  as 
nearly  square  as  possible. 

When  an  economizer  is  used,  the  breeching  and 
uptake  connections  should  be  larger  for  the  same 
total  draft  and  the  same  weight  of  coal  to  be 
burned  per  hour  than  when  an  economizer  is  not 
used,  because  about  one-third  of  the  total  draft 
will  be  used  to  overcome  the  friction  due  to  the 
economizer.  A  safe  rule  and  in  fact  one  that  will 
probably  give  sizes  a  little  large,  is  to  proportion 
the  breeching  and  uptake  connections  when  an 


122  MECHANICAL   DRAFT. 

economizer  is  used,  for  a  draft  that  is  two-thirds 
of  the  total  draft. 

Inlet  Chamber.  When  two  fans  are  installed 
in  a  mechanical  draft  plant  as  is  usual,  the  breech- 
ing from  the  boiler  is  lead  into  a  chamber,  called 
the  "  inlet  chamber,"  which  is  usually  placed  be- 
tween the  two  fans,  and  from  which  they  draw 
the  gases  of  combustion.  This  chamber  should 
be  provided  with  a  heavy  damper  by  means  of 
which  the  inlet  of  either  fan  may  be  closed,  thus 
putting  that  particular  fan  out  of  service  by 
preventing  the  gases  from  entering  it. 

Provision  should  always  be  made  for  fastening 
the  damper-  in  mid-position,  so  that  both  fans 
may  handle  4gases,  or  in  such  a  position  as  to  shut 
off  either  fan. 

Discharge  Chimney.  In  the  case  of  a  closed 
ash-pit  system  of  mechanical  draft  the  chimney 
should  be  high  enough  to  give  a  draft  not  less 
than  0.4  of  the  total  draft  required  for  the  com- 
bustion of  the  given  amount  of  coal  at  the  de- 
sired rate  in  pounds  per  square  foot  per  hour, 
and  the  cross-section  of  the  chimney  must  be 
sufficient  to  discharge  the  gases  under  this  draft. 

In  the  case  of  an  induced  system  of  mechanical 
draft  the  discharge  chimney  need  not  be  any 


PROPORTIONING    THE    PARTS. 


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124  MECHANICAL    DRAFT. 

higher  than  is  absolutely  necessary  to  discharge 
the  products  of  combustion  into  the  atmosphere 
above  the  surrounding  objects  or  buildings  which 
might  be  injured  by  the  hot  gases.  The  area  of 
the  cross-section  of  the  discharge  chimney  should 
not  be  less  than  the  area  of  a  circle  whose  di- 
ameter is  equal  to  one-half  the  diameter  of  the 
wheel.  That  is  to  say,  the  diameter  of  the  dis- 
charge chimney  should  not  be  less  than  one-half 
the  diameter  of  the  wheel,  and  it  must  be  made 
greater  the  higher  is  the  chimney. 

Where  two  fans  are  to  be  run  together  the 
diameter  of  the  discharge  chimney  or  stack  of  an 
induced  draft  apparatus  should  not  be  less  than 
0.7  of  the  diameter  of  the  fan  wheel. 

The  connections  between  the  fans  and  the  dis- 
charge chimney  should  be  as  short  and  as  straight 
as  possible  in  order  to  avoid  friction. 

By-pass.  When  an  induced  system  of  me- 
chanical draft  is  installed,  it  may  be  necessary  to 
put  in  a  "  by-pass  "  or  passage  through  which  the 
gases  may  pass  from  the  uptake  into  the  chimney 
without  passing  through  the  fan.  This  by-pass  is 
not  always  necessary,  although  in  large  plants  it 
is  customary  to  put  it  in.  It  is  never  made  large 
enough  to  take  all  the  gases  of  the  plant  when 


PROPORTIONING    THE    PARTS.  125 

running  at  its  full  capacity,  but  is  usually  of  such 
a  size  that  it  will  take  the  gases  from  the  plant 
when  running  at  about  5  or  10  per  cent  of  its 
capacity.  The  object  of  it  is  to  enable  steam  to 
be  raised  in  one  of  the  boilers  of  the  plant  so 
that  the  engines  which  are  to  drive  the  fans  may 
be  run. 

If  the  connections  between  the  boilers  and  the 
fan  are  short  and  the  discharge  chimney  is  not 
high,  there  is  then  no  necessity  for  a  by-pass,  as 
steam  can  usually  be  gotten  up  in  one  boiler  of 
the  plant,  at  least,  without  a  by-pass.  The  pres- 
sure to  which  the  steam  in  the  boiler  must  be 
raised  before  the  engine  driving  the  fan  can  be 
run  will  depend  of  course  upon  the  relative  size 
of  the  fan  and  the  engine  driving  it.  If  the  en- 
gine is  fairly  large  in  proportion  to  the  diameter 
of  the  fan  wheel,  there  ought  to  be  no  trouble  in 
starting  the  fan  when  the  steam  pressure  is  in 
the  neighborhood  of  20  or  25  pounds  by  the 
gage.  This  pressure,  of  course,  will  not  be  suffi- 
cient to  run  the  fan  at  its  full  capacity,  but  it 
will  be  usually  sufficient  to  start  the  fan ;  and  as 
the  fan  works,  the  fire  under  the  boiler  will  burn 
more  rapidly,  and  the  pressure  in  the  boiler  will 
rise,  and  as  the  pressure  rises  the  fan  will  go 
faster.  When  the  fan  has  gotten  up  to  its  speed. 


126  MECHANICAL    DRAFT. 

the  fires  can  be  built  under  the  other  boilers  of 
the  plant,  and  in  this  way  the  whole  plant  may  be 
set  in  operation. 

When  a  by-pass  is  put  in  the  size  of  it  must  be 
determined  according  to  the  length  and  size  of 
the  various  connections  between  the  boilers  and 
the  fan,  and  will  depend  very  largely  upon  local 
conditions,  so  that  it  is  hardly  possible  to  give  a 
rule  for  proportioning  it.  It  is  one  of  those 
things  which  must  depend  largely  upon  the  judg- 
ment of  the  engineer. 

Water  for  Bearings.  In  the  case  of  an  in- 
duced draft  system  it  is  absolutely  necessary  to 
have  a  sufficient  quantity  of  water  circulating 
through  the  bearings  of  the  fans  in  order  to  keep 
them  cool  when  handling  the  hot  gases.  It  is 
almost  impossible  to  predict  the  amount  of  water 
that  will  be  used,  as  it  depends  upon  the  tempera- 
ture of  the  gases  and  the  temperature  of  the  cool- 
ing1 water.  In  every  case,  however,  it  is  abso- 
lutely necessary  that  the  bearings  be  kept  cool, 
what  ever  amount  of  water  may  be  required. 

There  should  be  one  supply  pipe  and  one  re- 
turn pipe  run  to  each  bearing,  and  as  there  will 
be  one  water- jacketed  bearing  to  each  fan  that 
means  two  water  pipes  for  each  fan.  Each  of 


PROPORTIONING    THE    PARTS.  I2/ 

these  pipes  should  be  ordinary  j/2-inch  galvanized 
pipe,  and  each  should  be  provided  with  a  valve 
so  that  any  one  bearing  can  be  disconnected  from 
the  water  system  without  interfering  with  the 
other.  If  the  water  can  be  utilized  after  it  has 
passed  through  the  bearings  this  should  be  done : 
but  if  it  cannot  be  utilized  then  it  must  be  allowed 
to  go  to  waste,  and  the  expense  of  the  water  so 
wasted  must  be  considered  as  one  of  the  operating 
expenses  of  the  fan. 


APPENDIX, 


GENERAL  TABLES. 


129 


APPENDIX. 


°- — I 


i 


TABLE  XXIV. 


Dimensions  of  Full  Housed,  Top,  Horizontal  Discharge 
Fans. 


Diam- 
eter of 
wheel 
in  feet. 

Dimensions  of  housings  in  inches. 

I 

0 

G 

T 

J 

L 

Q 

3 

25 

18 

52 

61 

30 

22 

27 

3/2 

30 

21 

61 

7i 

36 

25 

3i 

4 

34 

24 

69 

81 

40 

29 

36 

4/2 

38 

27 

78 

9i 

45 

33 

40 

5 

42 

30 

87 

IOI 

50 

37 

44 

5/2 

47 

33 

95 

no 

55 

40 

49 

6 

5i 

36 

104 

120 

60 

44 

53 

6/2 

55 

39 

H3 

130 

65 

48 

57 

7 

59 

42 

121 

I4O 

70 

51 

61 

8 

68 

48 

139 

1  60 

80 

59 

70 

9 

76 

54 

156 

180 

90 

66 

79 

10 

85 

60 

173 

200 

ICO 

73 

87 

ii 

93 

66 

IQI 

22O 

no 

81 

96 

12 

102 

72 

208 

240 

1  20 

88 

105 

T32 


MECHANICAL   DRAFT. 


TABLE  XXV. 

Thickness  of  Black  Sheet  Iron  and  Steel  Usually  Used 
for  Breeching,  Uptakes,  Stacks,  etc.,  in  Connection 
with  Mechanical  Draft  Installations. 


United    States 

Thickness 

standard 

in   decimals 

gauge.* 

of   an    inch. 

8 

0.171875 

10 

0.140625 

12 

o.  109375 

14 

0.078125 

16 

0.0625 

*  Legalized  by  Congress,  March,  1893,  as  a  standard 
gage  for  sheet  and  plate  iron  and  steel.  It  is  used 
by  the  Custom  House  and  by  most  manufacturers  of 
sheet  iron  and  steel. 


APPENDIX. 


133 


TABLE  XXVI. 

Areas  of  Circles  from  10  Inches  to  72  Inches  in  Diame- 
ter, given  in   Square  Inches  to  the  nearest  Inch. 


Diam- 
eter. 

Area. 

Diam- 
eter. 

Area. 

Diam- 
eter. 

Area. 

10 

79 

3i 

755 

52 

2124 

ii 

95 

32 

804 

53 

2206 

12 

H3 

33 

855 

54 

2290 

13 

133 

34 

908 

55 

2376 

14 

154 

35 

962 

56 

2463 

15 

177 

36 

1018 

57 

2552 

16 

2OT 

37 

1075 

58 

2642 

17 

227 

38 

H34 

59 

2734 

18 

254 

39 

1195 

60 

2827 

19 

284 

40 

1257 

61 

2922 

20 

314 

4i 

1320 

62 

3019 

21 

346 

42 

1385 

63 

3H7 

22 

380 

43 

1452 

64 

3217 

23 

415 

44 

1521 

65 

33i8 

•   24 

452 

-  45- 

I5QO 

66 

3421 

25 

491 

46 

1662 

67 

3526 

26 

531 

47 

1735 

68 

3632 

27 

573 

48 

1810 

69 

3739 

28 

616 

49 

1886 

70 

3848 

29 

661 

50 

1964 

7i 

3959 

30 

707 

5i 

2043 

72 

4072 

134 


MECHANICAL   DRAFT. 


TABLE  XXVII. 

Sizes    of    Engines    Suitable    for    use    with    Mechanical 
Draft  Installations. 


Vertical. 

Horizontal. 

Diameter 

Stroke 

Diameter 

Stroke 

inches 

inches 

Id'2 

inches 

inches 

d 

/ 

d 

/ 

Id'2 

3 

4               35 

7 

10 

490 

4 

5 

80 

8 

10 

640 

5 

7 

175 

9 

12 

976 

6 

7 

252 

10 

12 

1  200 

7 

8 

392 

10 

14 

1400 

8 

8 

512 

ii 

14 

1694 

9 

9 

729 

12 

16 

2304 

9 

10 

810 

13 

16 

2704 

10 

10 

1000 

14 

18 

3528 

II 

10 

I2IO 

16 

20 

5120 

INDEX. 


PAGE 

AIR,  volume  of 47 

weight    required    . .  . . 46 

Ash-pit,   closed,   system 22 

BEARINGS,  water  for 126 

Breeching 117 

CAPACITY  of  fan 89 

Circles,   areas   of, 133 

Chimney,   discharge 122 

Chimney  and  mechanical  draft,  depreciation  and  re- 
pairs       12 

first  cost 7 

liability  to  derangement 6 

running  expenses 12 

Chimney  vs.  mechanical  draft 5 

Coal,  evaporation  per  pound  of 42 

fan  required  for  combustion  of 92 

weight  to  be  burned 41 

Combustion,  effect  of  rate  of 43 

rate  of,  relation  to  draft 52 

value  of  k 60 

Cylinder  for  engine 107 

135 


136  INDEX. 

PAGE 

DRAFT,  capacity  of  fan  for. 81 

chimney     i 

high     17 

mechanical     i 

relation  to  rate  of  combustion 52 

required   for  coals 60 

required  under  different  conditions 66 

required  for  different  rates  of  combustion 52 

resistance  due  to  economizer 63 

resistance  of  grate 60 

revolutions  of   fan 97 

ECONOMY  of  operation 14 

Economizers,  and  mechanical  draft 18 

cost  74 

decrease  of  temperature  of  gases 78 

effect  of  adding ' 70 

•effect  on  draft 63 

experiments  by  Roney 77 

increase  of  temperature  of  feed  water 75 

proportions,  ordinary 74 

resistance  due  to 63 

Engine,  size  required . . 107 

steam  used  by 1 1 1 

table  of  sizes  of . .  . . : 134 

Evaporation,  effect  of  rate  of 43 

maximum  per  pound  of  coal .' 42 

FACTOR   of   safety 51 

Fans,  bearings  for  induced  draft 83 

capacity    of 89 


INDEX.  137 
PAGE 

Fans,  capacity  for  forced  draft 91 

capacity  for   induced   draft 91 

'   centrifugal     '. 81 

choosing     in 

choosing  for    forced    draft 114 

choosing  for  induced  draft  with  economizers.  .  115 

choosing  for  induced  draft  without  economizers  116 

diameter     required 92 

dimensions    of    housings.  .  . 123 

draft  produced   by 8  r 

draft    and    revolutions    of 86 

horse-power    required 98 

how     designated 85 

inlet    of 86 

location    of 117 

pressure  produced   by 87 

required   for   coal 93 

size  of  engine  required 107 

size  for  forced  draft 94 

size  for  induced   draft  with  economizers 95 

size  for  induced  draft  without  economizers....  96 

speed    of 97 

speed  for  forced    draft 97 

speed  for  induced   draft   with   economizers q8 

speed  for  induced  draft  without  economizers..  98 

types  of 8 1 

Feed  water,  increase  of  temperature  by  economizer  75 

Fire-room,  system  of  closed 21 

Forced    draft,    advantages 26 

disadvantages    27 

economizer  with    26. 


138  INDEX. 

PAGE 

Forced  draft,  fan  for 23 

horse-power  required  for 104 

pressure   necessary 68 

size  of  fan  for 94 

speed  of  fan  for 99 

systems    of 20 

usual  pressure  of  air 24 

GASES,  decrease  of  temperature  by  economizer 79 

temperature    of 3 

volume  of 47 

volume  to  be  handled 48 

Goss,   tests  by 58 

Grate,  resistance  of. 60 

HORSE-POWER    98 

Mutton,   on   draft 54 

INCREASE  of  plant 16 

Induced  draft,  advantages 35 

disadvantages  . .  38 

economizer  with 37 

introduction 30 

temperature  cf  gases 32 

Induced  draft  with  economizer,  draft  necessary....  67 

horse-power  required  for  fan 105 

size  of  fan  for 95 

speed  of  fan  for 100 

Induced  draft  without  economizer,  draft  necessary.  .  66 

horse-power  required  for  fan 106 

size  of  fan  for 96 


INDEX.  139 

PAGE 

Induced  draft,  without  economizer,  speed  for  fan  for  101 

Inlet    chamber ; . .  . . 122 

Introduction i 

Iron  and  steel,  thickness  of 132 

LEAKAGE  of  air 50 

MECHANICAL  draft,   where   used 4 

systems    of 4 

SPEED  of   fan 97 

Steam    jets 3 

Steam  used  by  fan  engine 1 1 1 

TESTS,  by  Goss 58 

by  Wagner 59 

by  Whitham 56 

Thurston,   on   draft 55 

UPTAKE 117 

WATER  for  bearings 126 

Wagner,  tests  by 59 

Whitham,    tests    by 56 


INDEX  TO  TABLES. 


PAGE 

Breeching  and  uptake  connections  for  induced  draft, 

Table   XXIII 123 

Circles,  areas  of,  Table  XXVI 133 

Combustion,  rate  of,  Wagner's  tests,  Table  VI 60 

Whitham's  tests,  Tables  IV  and  V 57,  58 

Economizers,  factor  used  in  calculating,  Table  XII     79 
Roney's   experiments,   Table  XI 77 

Evaporation,  maximum  for  different  coals.  Table  I     44 
value  of  z  for  different  boilers,  Table  II 45 

Engine,   ratio   diameter  of   cylinder  to   diameter  of 

wheel,  Table  XXII 110 

sizes  of,  Table  XXVII 134 

Factor,  value  of  volume,  Table  III 48 

Fans,    dimensions    of   full    housed,    top,    horizontal 

discharge,  Table  XXIV 131 

141 


142  INDEX. 

PAGE 

Forced  draft,  fan  necessary,  Table  XIII 94 

power  required  for  fan,  Table  XIX 104 

pressure  necessary  in  ash-pit,  Table  X 69 

revolutions  of  fan,  Table  XVI. . .,. .  . . . . .- 99 

Furnace  resistance,  Whitham's  tests,  Table  VII ....  62 

Induced    draft    with    economizer,    draft    necessary, 

Table    IX 67 

fan  necessary,  Table  XIV 95 

power  required  for  fan,  Table  XX 105 

revolutions  of  fan,  Table  XVII too 

Induced  draft  without  economizer,  draft  necessary, 

'  Table  VIII 66 

fan  necessary,  Table  XV 96 

power  required  for  fan,  Table  XXI 106 

revolutions  of  fan,  Table  XVIII 101 

Iron,  thickness  of,  Table  XXV \V 


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